Compositions for enhancing the pluripotency of stem cells

ABSTRACT

Described herein is the finding that increasing the frequency of Zscan4 activation in mouse ES cells not only enhances, but also maintains their developmental potency in long-term cell culture. Particularly disclosed herein is the finding that the constitutive presence of Zscan4-ERT2, even in the absence of its usual activator tamoxifen, can increase the frequency of endogenous Zscan4 activation in ES cells, resulting in the increase of developmental potency of the ES cells. Accordingly, provided herein are Zscan4-ERT2 fusion proteins and nucleic acid molecules and vectors encoding Zscan4-ERT2 fusion proteins. Further provided are methods of prolonging and/or enhancing stem cell plmipotency using the disclosed Zscan4-ERT2 nucleic acid molecules and fusion proteins.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase application of PCT/US2012/030005, filed Mar. 21, 2012, which claims priority to U.S. Provisional Application Ser. No. 61/466,667 filed Mar. 23, 2011, the contents of which are hereby incorporated by reference in the present disclosure in their entirety.

SUBMISSION OF SEQUENCE LISTING ON ASCII TEXT FILE

The content of the following submission on ASCII text file is incorporated herein by reference in its entirety: a computer readable form (CRF) of the Sequence Listing (file name: 699442000400SeqList.txt, date recorded: Sep. 23, 2013, size: 101 KB).

FIELD

This disclosure concerns compositions and methods for enhancing or prolonging the pluripotency of a stem cell, and the use of such pluripotent stem cells.

BACKGROUND

Mouse embryonic stem (ES) cells are prototypical pluripotent cells, which are derived from the inner cell mass of blastocysts (Martin, Proc Natl Acad Sci USA 78:7634-7638, 1981; Evans and Kaufman, Nature 292:154-156, 1981). ES cells have an unusual capacity of proliferating for a long time without losing their genome integrity and karyotype (Suda et al., J Cell Physiol 133:197-201, 1987), and are capable of contributing to all the cell types in animals upon injection into mouse blastocysts (Niwa, Development 134:635-646, 2007). The most striking evidence of their potency has been demonstrated by injecting ES cells into tetraploid (4N) blastocysts, which produces healthy pups entirely from ES cells (Nagy et al., Proc Natl Acad Sci USA 90:8424-8428, 1993). The ultimate test was to see if a single ES cell can form an entire healthy pup, though the success rate was extremely low (0.5%) (Wang and Jaenisch, Dev Biol 275:192-201, 2004).

It has recently been shown that Zscan4 (Zinc finger and scan domain-containing protein 4), which is expressed specifically in 2-cell stage embryos and ES cells (Falco et al., Dev Biol 307:539-550, 2007), is required for the maintenance of genome stability and normal karyotype in ES cells (Zalzman et al., Nature 464:858-863, 2010). Although only a small fraction (˜5%) of undifferentiated ES cells express Zscan4 at a given time (Falco et al., Dev Biol 307:539-550, 2007), essentially all of the ES cells in culture undergo the transient Zscan4⁺ state within 9 passages (Zalzman et al., Nature 464:858-863, 2010). Upon short hairpin RNA (shRNA)-mediated repression of Zscan4, after about 8 passages ES cells undergo massive karyotype deterioration. Prior studies have also shown that the Zscan4⁺ state of ES cells is associated with telomere extension (Zalzman et al., Nature 464:858-863, 2010). Although ES cells have the best capacity to maintain their genome integrity in culture, it is also widely recognized that even ES cells, in long-term culture, gradually lose their developmental potency (i.e., ability to contribute to tissues in chimeric mice).

SUMMARY

Disclosed herein is the finding that increasing the frequency of Zscan4 activation in mouse ES cells not only enhances, but maintains their developmental potency in long-term cell culture. In particular, disclosed herein is the finding that particular Zscan4 protein truncations and fusion proteins increase the number of Zscan4⁺ cells and/or promote recurrent activation of Zscan4 in stem cells.

Provided herein are nucleic acid molecules, including vectors, encoding a Zscan4-ERT2 fusion protein. Recombinant Zscan4-ERT2 fusion proteins are also provided. Compositions and cells (such as ES cell or iPS cells) comprising the Zscan4-ERT2 nucleic acid molecules and fusion proteins are also provided herein.

Further provided are nucleic acid molecules, including vectors, encoding a Zscan4 protein with a C-terminal truncation of at least one zinc finger domain, referred herein to as Zscan4-ΔC. Recombinant Zscan4-ΔC proteins are also provided. Compositions and cells (such as ES cell or iPS cells) comprising the Zscan4-ΔC nucleic acid molecules and proteins are also provided herein.

Further provided are methods of enhancing or prolonging the pluripotency of a stem cell or a stem cell population; methods of increasing the frequency of Zscan4 positive cells in a stem cell population; and methods of promoting genome stability or increasing telomere length, or both, in a stem cell or a stem cell population, by increasing the frequency of Zscan4 activation in the stem cell or stem cell population. In some embodiments, the methods include contacting the stem cell or stem cell population with a Zscan4-ERT2 nucleic acid molecule, fusion protein or composition as disclosed herein. In other embodiments, the methods include contacting the stem cell or stem cell population with a Zscan4-ΔC nucleic acid molecule, protein or composition as disclosed herein.

The foregoing and other objects and features of the disclosure will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F: Constitutive expression of a Zscan4c-ERT2 fusion protein increases the number of Zscan4⁺ ES cells. FIG. 1A is a schematic of the structure of a Zscan4c-ERT2 fusion protein. Zscan4c contains one SCAN domain and four C2H2 zinc finger domains. FIG. 1B are fluorescence microscopy images of MC1-ZE3 cells, in which a Zscan4 promoter drives the expression of Emerald marker (left), MC1-ZE3ZERT2 clone #15 cells, in which the Zscan4c-ERT2 fusion protein is constitutively expressed, cultured in the absence of Tmx (middle), and MC1-ZE3-ZERT2 clone #15 cells cultured in the presence of Tmx for 3 days (right). FIG. 1C is a graph showing flow-cytometry analysis of MC1-ZE3 ES cells (left, control) and MC1-ZE3-ZERT2 #15 ES cells (right) in the absence or presence of 1 μM Tmx. Em fluorescence levels (average ±S.E.M.; n=6) are shown. Note 3-fold increase of Em⁺ cells by the constitute expression of a Zscan4c-ERT2 fusion protein even without Tmx. FIG. 1D is a graph showing the results of quantitative RT-PCR analysis of endogenous Zscan4 expression measured by using PCR primer pairs specific for 3′-UTR of Zscan4 in MC1-ZE3 ES cells (left, control) and MC1-ZE3-ZERT2 #15 ES cells (right) in the absence or presence of 1 μM Tmx. The fold-induction of endogenous Zscan4 expression levels (average ±S.E.M.; n=6) compared to that of control MC1-ZE3 is shown. Note the 6 fold increase of endogenous Zscan4 at the RNA level by the constitute expression of a Zscan4c-ERT2 fusion protein even without Tmx. FIG. 1E is a series of images of V6.5 parental ES cells (passage number 14), V6.5 ZERT2 #2 (p.20), V6.5 ZERT2 #7 (p.21), V6.5 ZERT2 #10 (p.20), V6.5 ZERT2 #18 (p.22) ES cell colonies after whole-mount RNA in situ hybridization of a Zscan4 full-length probe, which detects both endogenous and exogenous Zscan4 RNAs (upper panel) or a Zscan4 3′-UTR probe, which detects only endogenous Zscan4 RNAs (lower panel). FIG. 1F is a schematic showing comparisons of global expression profiles between V6.5 ZERT2 #18 ES cells and Em⁺ ES cells (upper panel), and between Tmx⁻ and Tmx⁺ conditions of V6.5 ZERT2 #18 ES cells (lower panel). Note that Zscan4-related genes (Zscan4c, BC061212, Tmeme92, and Tcstv1/3) are already upregulated in the V6.5 ZERT2 #18 ES cells even without Tmx.

FIGS. 2A-2G: Zscan4 lacking the C-terminus increases the number of Zscan4⁺ cells. FIG. 2A is a schematic showing the structure of Zscan4c, Zscan4cERT2, Zscan4c-ΔC and Zscan4c-ΔN proteins. Zscan4c-ΔC was made by deleting four Zinc finger domains at the C-terminus of Zscan4c protein. Zscan4c-ΔN was made by deleting the SCAN domain at the N-terminus. These mutated genes were placed under the strong and constitutive CAG promoter. Each vector was transfected into MC1ZE16 ES cells (sister clones of MC1-ZE3). Multiple independent clones were isolated: ZDC-MC1-ZE16 #3, #4, #20 for Zscan4c-ΔC; ZDN-MC1-ZE16 #5, #8, #15 for Zscan4c-ΔN. FIGS. 2B-2G are fluorescence microscopic images of ZDC-MC1-ZE16 #3, #4, #20 for Zscan4c-ΔC and ZDN-MC1-ZE16 #5, #8, #15 for Zscan4c-ΔN. The results demonstrate that the expression of Zscan4c-ΔC increases the number of Zscan4⁺ cells, whereas the expression of Zscan4c-ΔN does not change the number of Zscan4⁺ cells.

FIGS. 3A-3B: Constitutive expression of a Zscan4c-ERT2 fusion protein increases and prolongs developmental potency of ES cells. FIG. 3A shows representative coat colors of chimeric mice generated by injecting various ES cells into blastocysts. The higher chimerism represents the higher contribution of injected ES cells to mice, indicating the higher developmental potency of ES cells. FIG. 3B is a graph showing the percent distribution of chimerism levels among “n” number of mice born from various ES cell lines.

FIGS. 4A-4E: Tetraploid (4N) complementation assays confirm the higher potency of ES cells expressing a Zscan4c-ERT2 fusion protein. FIG. 4A is a table showing development of 4N blastocysts injected with multiple ES cells (10-15 ES cells): V6.5 parental ES cells (passage 18), V6.5 ZERT2 #7 (passage 22), V6.5 ZERT2 #10 (passage 22), V6.5 ZERT2 #18 (passage 19), and freshly isolated TA1 ES cells (passage 3). FIG. 4B is a table showing development of 4N blastocysts injected with single ES cells: V6.5 parental ES cells (passage 18), V6.5 ZERT2 #2 (passage 21), V6.5 ZERT2 #18, and freshly isolated TA1 ES cells (passage 4). FIG. 4C is an image of the embryos examined: only properly developed embryos were counted (the group on the right). FIG. 4D is a pair of images of two live embryos derived from single V6.5 ZERT2 #18 ES cells shown in FIG. 4A. FIG. 4E shows a proposed model of ES cell potency.

FIGS. 5A-5C are a table providing a list of genes upregulated in MC1-ZE7 Em⁺ cells compared to Em⁻ cells.

FIGS. 6A-6C: Generation and characterization of V6.5 ZERT2 ES cell clones. FIG. 6A is a graph showing results of qRT-PCR analysis of Zscan4 expression levels by a primer pair detecting RNA from both endogenous Zscan4 and exogenous Zscan4 (transcripts from a pCGA-Zscan4-ERT2). The primer sequences are 5′-AGTCTGACTGATGAGTGCTTGAAGCC-3′ (SEQ ID NO: 15) and 5′-GGCCTTGTTGCAGATTGCTGTTG-3′ (SEQ ID NO: 16). Data were normalized by the expression of H2A, using primers 5′-TTGCAGCTTGCTATACGTGGAGATG-3′ (SEQ ID NO: 17) and 5′-TGTTGTCCTTTCTTCCCGATCAGC-3′ (SEQ ID NO: 18). The expression levels are shown as a fold change relative to the Zscan4 expression levels of a parental V6.5 ES cells. FIG. 6B is a graph of growth curves of V6.5 ZERT2 #18 ES cells in the presence (Tmx+) or absence of Tamoxifen (Tmx−). The presence of Tmx dramatically reduced the proliferation of ES cells, which was restored by removing the Tmx from the media even after long-term culture with Tmx (Tmx+>−). FIG. 6C is a series of images showing morphologies of cells in each culture condition.

FIG. 7A is a scatter plot showing genes expressed differentially between V 6.5 ZERT2 #18 ES cells and control V6.5 #2 ES cells. FIG. 7B is a scatter plot showing genes expressed differentially between V6.5 ZERT2 #18 ES cells cultured for 2 days in the presence of Tmx and control V6.5 #2 ES cells.

FIG. 8 is a table listing the top 50 genes upregulated in V6.5 ZERT2 #18 ES cells compared to V6.5 #2 ES cells.

FIG. 9 is a table listing the top 50 genes upregulated in V6.5 ZERT2 #18 ES cells cultured in the presence of Tmx for 2 days compared to V6.5 #2 ES cells.

FIGS. 10A-10B: Derivation of new F1 (C57BL/6J×12956/SvEvTac) hybrid ES cell lines. FIG. 10A is a table showing blastocysts obtained by mating C57BL/6J females with 12956/SvEvTac males. Blastocysts were cultured in vitro on the mouse embryo fibroblasts (MEFs) feeders in 15% KSR medium (Invitrogen) supplemented with 50 nM PD98059 (MEK1 inhibitor). *Outgrowths showed undifferentiated (U), differentiated (D), and mixed (U/D) cellular phenotypes. FIG. 10B is table providing a summary of ES derivation results.

FIGS. 11A-11B: Testing developmental potency of newly derived F1 hybrid ES cell lines by tetraploid complementation assays. FIG. 11A is a table of six ES cell lines that showed undifferentiated cellular phenotypes when injected into tetraploid (4N) mouse blastocysts. Success rates of obtaining live embryos at E13.5 varied among ES cell lines, ranging from 15% to 60%. Clone #10 was selected for its highest success rate (named TA1 ES cell line) and was used for subsequent studies. FIG. 11B is a series of representative images of 4N placentas and E13.5 embryos derived from the TA1 ES cell line. Normal appearance of female and male gonads dissected from these embryos indicates their germline competence.

FIG. 12: Testing transient overexpression of Zscan4 (i.e., unmodified Zscan4. FIG. 12 includes a graph showing that the transient overexpression of Zscan4 was able to increase the developmental potency of ES cells.

SEQUENCE LISTING

The nucleic and amino acid sequences listed in the accompanying sequence listing are shown using standard letter abbreviations for nucleotide bases, and three letter code for amino acids, as defined in 37 C.F.R. 1.822. Only one strand of each nucleic acid sequence is shown, but the complementary strand is understood as included by any reference to the displayed strand. In the accompanying sequence listing:

SEQ ID NOs: 1 and 2 are nucleotide and amino acid sequences of human ZSCAN4.

SEQ ID NOs: 3 and 4 are nucleotide and amino acid sequences of mouse Zscan4a.

SEQ ID NOs: 5 and 6 are nucleotide and amino acid sequences of mouse Zscan4b.

SEQ ID NOs: 7 and 8 are nucleotide and amino acid sequences of mouse Zscan4c.

SEQ ID NOs: 9 and 10 are nucleotide and amino acid sequences of mouse Zscan4d.

SEQ ID NOs: 11 and 12 are nucleotide and amino acid sequences of mouse Zscan4e.

SEQ ID NOs: 13 and 14 are nucleotide and amino acid sequences of mouse Zscan4f.

SEQ ID NOs: 15-18 are nucleotide sequences of primers used for qRT-PCR analysis of Zscan4 and H2A.

SEQ ID NO: 19 is the nucleotide acid sequence of plasmid pPyCAGmZscan4c-ERT2.

SEQ ID NO: 20 is the nucleotide sequence of plasmid pPyCAG-hZscan4ERT2.

SEQ ID NO: 21 is the amino acid sequence of human ERT2.

SEQ ID NO: 22 is the amino acid sequence of a mouse Zscan4c-ERT2 fusion protein.

SEQ ID NO: 23 is the amino acid sequence of a human ZSCAN4-ERT2 fusion protein.

SEQ ID NO: 24 is the nucleotide sequence of plasmid pCAG-Zscan4-ΔC.

SEQ ID NO: 25 is the amino acid sequence of mouse Zscan4c-ΔC (lacking all four zinc finger domains).

DETAILED DESCRIPTION I. Abbreviations

-   -   a.a. amino acid     -   cDNA complementary deoxyribonucleic acid     -   Em Emerald     -   ES embryonic stem     -   hCG human chorionic gonadotropin     -   ICM inner cell mass     -   LIF leukemia inhibitory factor     -   MEF murine embryonic fibroblast     -   ORF open reading frame     -   PFA paraformaldehyde     -   qPCR quantitative polymerase chain reaction     -   qRT-PCR quantitative reverse transcriptase polymerase chain         reaction     -   shRNA short hairpin ribonucleic acid     -   Tmx tamoxifen

II. Terms and Methods

Unless otherwise noted, technical terms are used according to conventional usage.

Definitions of common terms in molecular biology may be found in Benjamin Lewin, Genes V, published by Oxford University Press, 1994 (ISBN 0-19-854287-9); Kendrew et al. (eds.), The Encyclopedia of Molecular Biology, published by Blackwell Science Ltd., 1994 (ISBN 0-632-02182-9); and Robert A. Meyers (ed.), Molecular Biology and Biotechnology: a Comprehensive Desk Reference, published by VCH Publishers, Inc., 1995 (ISBN 1-56081-569-8). In order to facilitate review of the various embodiments of the disclosure, the following explanations of specific terms are provided:

Administration:

To provide or give a subject an agent, such as an ES cell or population of ES cells, by any effective route. An exemplary route of administration includes, but is not limited to, injection (such as subcutaneous, intramuscular, intradermal, intraperitoneal, intravenous or intra-arterial).

Agent:

Any protein, nucleic acid molecule, compound, cell, small molecule, organic compound, inorganic compound, or other molecule of interest. Contacting: Placement in direct physical association; includes both in solid and liquid form. As used herein, “contacting” is used interchangeably with “exposed.” In some cases, “contacting” includes transfecting, such as transfecting a nucleic acid molecule into a cell.

Degenerate Variant:

A polynucleotide encoding a polypeptide, such as a Zscan4 polypeptide, that includes a sequence that is degenerate as a result of the genetic code. There are 20 natural amino acids, most of which are specified by more than one codon. Therefore, all degenerate nucleotide sequences are included as long as the amino acid sequence of the polypeptide encoded by the nucleotide sequence is unchanged.

Differentiation:

Refers to the process by which a cell develops into a specific type of cell (for example, muscle cell, skin cell etc.). Differentiation of embryonic stem cells refers to the development of the cells toward a specific cell lineage. As a cell becomes more differentiated, the cell loses potency, or the ability to become multiple different cell types.

Embryonic Stem (ES) Cells:

Pluripotent cells isolated from the inner cell mass of a developing blastocyst. ES cells can be derived from any organism, such as a mammal. In one embodiment, ES cells are produced from mice, rats, rabbits, guinea pigs, goats, pigs, cows, non-human primates or humans. Human and murine derived ES cells are exemplary. ES cells are pluripotent cells, meaning that they can generate all of the cells present in the body (bone, muscle, brain cells, etc.). Methods for producing murine ES cells can be found, for example, in U.S. Pat. No. 5,670,372. Methods for producing human ES cells can be found, for example, in U.S. Pat. No. 6,090,622, PCT Publication No. WO 00/70021 and PCT Publication No. WO 00/27995. A number of human ES cell lines are known in the art and are publically available. For example, the National Institutes of Health (NIH) Human Embryonic Stem Cell Registry provides a list of a number of human ES cell lines that have been developed (a list can be found online at the NIH Office of Extramural Research website (http://grants.nih.gov/stem_cells/registry/current.htm).

Encapsulated:

As used herein, a molecule “encapsulated” in a nanoparticle refers to a molecule (such as Zscan4-ERT2 fusion protein) that is either contained within the nanoparticle or attached to the surface of the nanoparticle, or a combination thereof.

ERT2:

A protein comprising a mutated ligand binding domain of the human estrogen receptor that does not bind its natural ligand (17β-estradiol) at physiological concentrations, but is highly sensitive to nanomolar concentrations of tamoxifen or its metabolite 4-hydroxytamoxifen (4OHT) (Feil et al., Biochem Biophys Res Commun 237(3):752-757, 1997). An exemplary amino acid sequence for ERT2 is set forth herein as SEQ ID NO: 21, and the corresponding coding sequence is set forth herein as nucleotides 3989-4936 of SEQ ID NO: 19.

ES Cell Therapy:

A treatment that includes administration of ES cells to a subject. In particular examples, the ES cells are Zscan4⁺ ES cells.

Functional Fragment or Variant (of Zscan4):

The disclosed Zscan4 polynucleotides and polypeptides (such as those set forth as SEQ ID NOs: 1-14) include functional fragments and variants of Zscan4 that retain Zscan4 biological activity. Functional fragments and/or variants of Zscan4 generally comprise at least about 80%, at least about 85%, at least about 90%, at least about 95% or at least about 99% sequence identity with one of the Zscan4 sequences set forth as SEQ ID NOs 1-14. When less than the entire sequence is being compared for sequence identity, fragments will typically possess at least 80% sequence identity over the length of the fragment, and can possess, for example, sequence identities of at least 85%, 90%, 95% or 99%.

Fusion Protein:

A protein generated by expression of a nucleic acid sequence engineered from nucleic acid sequences encoding at least a portion of two different (heterologous) proteins. To create a fusion protein, the nucleic acid sequences must be in the same reading frame and contain no internal stop codons. In some embodiments herein, the fusion protein is a Zscan4-ERT2 fusion protein. In some examples, the fusion protein comprises a linker between the two different proteins.

Genome Stability:

The ability of a cell to faithfully replicate DNA and maintain integrity of the DNA replication machinery. An ES cell with a stable genome generally defies cellular senescence, can proliferate more than 250 doublings without undergoing crisis or transformation, has a low mutation frequency and a low frequency of chromosomal abnormalities (e.g., relative to embryonal carcinoma cells), and maintains genomic integrity. Long telomeres are thought to provide a buffer against cellular senescence and be generally indicative of genome stability and overall cell health. Chromosome stability (e.g., few mutations, no chromosomal rearrangements or change in number) is also associated with genome stability. A loss of genome stability is associated with cancer, neurological disorders and premature aging. Signs of genome instability include elevated mutation rates, gross chromosomal rearrangements, alterations in chromosome number, and shortening of telomeres.

Heterologous:

A heterologous polypeptide or polynucleotide refers to a polypeptide or polynucleotide derived from a different source or species. For example, a mouse Zscan4 peptide expressed in a human ES cell is heterologous to that ES cell.

Host Cells:

Cells in which a vector can be propagated and its DNA expressed. The term also includes any progeny of the subject host cell. It is understood that all progeny may not be identical to the parental cell since there may be mutations that occur during replication. However, such progeny are included when the term “host cell” is used.

Induced Pluripotent Stem (iPS) Cells:

A type of pluripotent stem cell artificially derived from a non-pluripotent cell, typically an adult somatic cell, by inducing a “forced” expression of certain genes. iPS cells can be derived from any organism, such as a mammal. In one embodiment, iPS cells are produced from mice, rats, rabbits, guinea pigs, goats, pigs, cows, non-human primates or humans. Human and murine derived iPS cells are exemplary.

iPS cells are similar to ES cells in many respects, such as the expression of certain stem cell genes and proteins, chromatin methylation patterns, doubling time, embryoid body formation, teratoma formation, viable chimera formation, and potency and differentiability. Methods for producing iPS cells are known in the art. For example, iPS cells are typically derived by transfection of certain stem cell-associated genes (such as Oct-3/4 (Pouf51) and Sox2) into non-pluripotent cells, such as adult fibroblasts. Transfection can be achieved through viral vectors, such as retroviruses, lentiviruses, or adenoviruses. For example, cells can be transfected with Oct3/4, Sox2, Klf4, and c-Myc using a retroviral system or with OCT4, SOX2, NANOG, and LIN28 using a lentiviral system. After 3-4 weeks, small numbers of transfected cells begin to become morphologically and biochemically similar to pluripotent stem cells, and are typically isolated through morphological selection, doubling time, or through a reporter gene and antibiotic selection. In one example, iPS from adult human cells are generated by the method of Yu et al. (Science 318(5854):1224, 2007) or Takahashi et al. (Cell 131(5):861-72, 2007).

Isolated:

An isolated nucleic acid has been substantially separated or purified away from other nucleic acid sequences and from the cell of the organism in which the nucleic acid naturally occurs, i.e., other chromosomal and extrachromosomal DNA and RNA. The term “isolated” thus encompasses nucleic acids purified by standard nucleic acid purification methods. The term also embraces nucleic acids prepared by recombinant expression in a host cell as well as chemically synthesized nucleic acids. Similarly, “isolated” proteins have been substantially separated or purified from other proteins of the cells of an organism in which the protein naturally occurs, and encompasses proteins prepared by recombination expression in a host cell as well as chemically synthesized proteins. Similarly, “isolated” cells have been substantially separated away from other cell types.

Linker:

One or more nucleotides or amino acids that serve as a spacer between two molecules, such as between two nucleic acid molecules or two peptides (such as in a fusion protein). In some examples a linker is 1 to 100 amino acids, such as 1 to 50 or 5 to 10 amino acids.

Nanoparticle:

A particle less than about 1000 nanometers (nm) in diameter. Exemplary nanoparticles for use with the methods provided herein are made of biocompatible and biodegradable polymeric materials. In some embodiments, the nanoparticles are PLGA nanoparticles. As used herein, a “polymeric nanoparticle” is a nanoparticle made up of repeating subunits of a particular substance or substances. “Poly(lactic acid) nanoparticles” are nanoparticles having repeated lactic acid subunits. Similarly, “poly(glycolic acid) nanoparticles” are nanoparticles having repeated glycolic acid subunits.

Non-Human Animal:

Includes all animals other than humans. A non-human animal includes, but is not limited to, a non-human primate, a farm animal such as swine, cattle, and poultry, a sport animal or pet such as dogs, cats, horses, hamsters, rodents, such as mice, or a zoo animal such as lions, tigers or bears. In one example, the non-human animal is a transgenic animal, such as a transgenic mouse, cow, sheep, or goat. In one specific, non-limiting example, the transgenic non-human animal is a mouse.

Operably Linked:

A first nucleic acid sequence is operably linked to a second nucleic acid sequence when the first nucleic acid sequence is placed in a functional relationship with the second nucleic acid sequence. For instance, a promoter is operably linked to a coding sequence if the promoter affects the transcription or expression of the coding sequence. Generally, operably linked nucleic acid sequences are contiguous and where necessary to join two protein coding regions, in the same reading frame.

Pharmaceutically Acceptable Carriers:

The pharmaceutically acceptable carriers of use are conventional. Remington's Pharmaceutical Sciences, by E. W. Martin, Mack Publishing Co., Easton, Pa., 15th Edition (1975), describes compositions and formulations suitable for pharmaceutical delivery of the Zscan4 proteins (including fusion proteins), Zscan4 nucleic acid molecules, or cells herein disclosed.

In general, the nature of the carrier will depend on the particular mode of administration being employed. For instance, parenteral formulations usually comprise injectable fluids that include pharmaceutically and physiologically acceptable fluids such as water, physiological saline, balanced salt solutions, aqueous dextrose, glycerol or the like as a vehicle. For solid compositions (e.g., powder, pill, tablet, or capsule forms), conventional non-toxic solid carriers can include, for example, pharmaceutical grades of mannitol, lactose, starch, or magnesium stearate. In addition to biologically-neutral carriers, pharmaceutical compositions to be administered can contain minor amounts of non-toxic auxiliary substances, such as wetting or emulsifying agents, preservatives, and pH buffering agents and the like, for example, sodium acetate or sorbitan monolaurate.

Pluripotent/Pluripotency:

A “pluripotent” cell is a cell that can form all of an organism's cell lineages (endoderm, mesoderm and ectoderm), including germ cells, but cannot form an entire organisms autonomously. As used herein, enhancing or prolonging pluripotency refers to increasing the pluripotent capacity or duration of pluripotency of a stem cell.

Polypeptide:

A polymer in which the monomers are amino acid residues which are joined together through amide bonds. When the amino acids are alpha-amino acids, either the L-optical isomer or the D-optical isomer can be used, the L-isomers being preferred. The terms “polypeptide” or “protein” as used herein are intended to encompass any amino acid sequence and include modified sequences such as glycoproteins. The term “polypeptide” is specifically intended to cover naturally occurring proteins, as well as those which are recombinantly or synthetically produced.

The term “polypeptide fragment” refers to a portion of a polypeptide which exhibits at least one useful epitope. The term “functional fragments of a polypeptide” refers to all fragments of a polypeptide that retain an activity of the polypeptide, such as a Zscan4. Biologically functional fragments, for example, can vary in size from a polypeptide fragment as small as an epitope capable of binding an antibody molecule to a large polypeptide capable of participating in the characteristic induction or programming of phenotypic changes within a cell, including affecting cell proliferation or differentiation. An “epitope” is a region of a polypeptide capable of binding an immunoglobulin generated in response to contact with an antigen. Thus, smaller peptides containing the biological activity of Zscan4, or conservative variants of Zscan4, are thus included as being of use.

The term “substantially purified polypeptide” as used herein refers to a polypeptide which is substantially free of other proteins, lipids, carbohydrates or other materials with which it is naturally associated. In one embodiment, the polypeptide is at least 50%, for example at least 80% free of other proteins, lipids, carbohydrates or other materials with which it is naturally associated. In another embodiment, the polypeptide is at least 90% free of other proteins, lipids, carbohydrates or other materials with which it is naturally associated. In yet another embodiment, the polypeptide is at least 95% free of other proteins, lipids, carbohydrates or other materials with which it is naturally associated.

Conservative substitutions replace one amino acid with another amino acid that is similar in size, hydrophobicity, etc. Examples of conservative substitutions are shown below:

Original Residue Conservative Substitutions Ala Ser Arg Lys Asn Gln; His Asp Glu Cys Ser Gln Asn Glu Asp His Asn; Gln Ile Leu; Val Leu Ile; Val Lys Arg; Gln; Glu Met Leu; Ile Phe Met; Leu; Tyr Ser Thr Thr Ser Trp Tyr Tyr Trp; Phe Val Ile; Leu

Variations in the cDNA sequence that result in amino acid changes, whether conservative or not, should be minimized in order to preserve the functional and immunologic identity of the encoded protein. Thus, in several non-limiting examples, a Zscan4 polypeptide (or Zscan4 fusion protein, such as Zscan4-ERT2), or other polypeptides disclosed herein, includes at most two, at most five, at most ten, at most twenty, or at most fifty conservative substitutions. The immunologic identity of the protein may be assessed by determining whether it is recognized by an antibody; a variant that is recognized by such an antibody is immunologically conserved. Any cDNA sequence variant will preferably introduce no more than twenty, and preferably fewer than ten amino acid substitutions into the encoded polypeptide. Variant amino acid sequences may be, for example, at least 80%, 90% or even 95% or 98% identical to the native amino acid sequence (such as a native Zscan4 sequence or a Zscan4-ERT2 sequence, such as SEQ ID NO: 22 or 23).

Promoter:

Nucleic acid control sequences which direct transcription of a nucleic acid. A promoter includes necessary nucleic acid sequences near the start site of transcription. A promoter also optionally includes distal enhancer or repressor elements. A “constitutive promoter” is a promoter that is continuously active and is not subject to regulation by external signals or molecules. In contrast, the activity of an “inducible promoter” is regulated by an external signal or molecule (for example, a transcription factor).

Recombinant:

A recombinant nucleic acid or polypeptide is one that has a sequence that is not naturally occurring or has a sequence that is made by an artificial combination of two otherwise separated segments of sequence. This artificial combination is often accomplished by chemical synthesis or by the artificial manipulation of isolated segments of nucleic acids, for example, by genetic engineering techniques.

Sequence Identity/Similarity:

The identity/similarity between two or more nucleic acid sequences, or two or more amino acid sequences, is expressed in terms of the identity or similarity between the sequences. Sequence identity can be measured in terms of percentage identity; the higher the percentage, the more identical the sequences are. Sequence similarity can be measured in terms of percentage similarity (which takes into account conservative amino acid substitutions); the higher the percentage, the more similar the sequences are. Homologs or orthologs of nucleic acid or amino acid sequences possess a relatively high degree of sequence identity/similarity when aligned using standard methods. This homology is more significant when the orthologous proteins or cDNAs are derived from species which are more closely related (such as human and mouse sequences), compared to species more distantly related (such as human and C. elegans sequences).

Methods of alignment of sequences for comparison are well known in the art. Various programs and alignment algorithms are described in: Smith & Waterman, Adv. Appl. Math. 2:482, 1981; Needleman & Wunsch, J. Mol. Biol. 48:443, 1970; Pearson & Lipman, Proc. Natl. Acad. Sci. USA 85:2444, 1988; Higgins & Sharp, Gene, 73:237-44, 1988; Higgins & Sharp, CABIOS 5:151-3, 1989; Corpet et al., Nuc. Acids Res. 16:1088190, 1988; Huang et al. Computer Appls. in the Biosciences 8, 155-65, 1992; and Pearson et al., Meth. Mol. Bio. 24:307-31, 1994. Altschul et al., J. Mol. Biol. 215:403-10, 1990, presents a detailed consideration of sequence alignment methods and homology calculations.

The NCBI Basic Local Alignment Search Tool (BLAST) (Altschul et al., J. Mol. Biol. 215:403-10, 1990) is available from several sources, including the National Center for Biological Information (NCBI, National Library of Medicine, Building 38A, Room 8N805, Bethesda, Md. 20894) and on the Internet, for use in connection with the sequence analysis programs blastp, blastn, blastx, tblastn and tblastx. Additional information can be found at the NCBI web site.

Stem Cell:

A cell having the unique capacity to produce unaltered daughter cells (self-renewal; cell division produces at least one daughter cell that is identical to the parent cell) and to give rise to specialized cell types (potency). Stem cells include, but are not limited to, ES cells, EG cells, GS cells, MAPCs, maGSCs, USSCs, adult stem cells and induced pluripotent stem cells. In one embodiment, stem cells can generate a fully differentiated functional cell of more than one given cell type. The role of stem cells in vivo is to replace cells that are destroyed during the normal life of an animal. Generally, stem cells can divide without limit. After division, the stem cell may remain as a stem cell, become a precursor cell, or proceed to terminal differentiation. A precursor cell is a cell that can generate a fully differentiated functional cell of at least one given cell type. Generally, precursor cells can divide. After division, a precursor cell can remain a precursor cell, or may proceed to terminal differentiation.

Subject:

Living multi-cellular vertebrate organisms, a category that includes human and non-human mammals.

Subpopulation:

An identifiable portion of a population. As used herein, a “subpopulation” of ES cells expressing Zscan4 is the portion of ES cells in a given population that has been identified as expressing Zscan4.

Telomere:

Refers to the end of a eukaryotic chromosome, a specialized structure involved in the replication and stability of the chromosome. Telomeres consist of many repeats of a short DNA sequence in a specific orientation. Telomere functions include protecting the ends of the chromosome so that chromosomes do not end up joined together, and allowing replication of the extreme ends of the chromosomes (by telomerase). The number of repeats of telomeric DNA at the end of a chromosome decreases with age.

Transfecting or Transfection:

Refers to the process of introducing nucleic acid into a cell or tissue. Transfection can be achieved by any one of a number of methods, such as, but not limited to, liposomal-mediated transfection, electroporation and injection.

Vector:

A nucleic acid molecule as introduced into a host cell, thereby producing a transformed host cell. A vector may include nucleic acid sequences that permit it to replicate in a host cell, such as an origin of replication (DNA sequences that participate in initiating DNA synthesis). For example, an expression vector contains the necessary regulatory sequences to allow transcription and translation of inserted gene or genes. A vector may also include one or more selectable marker genes and other genetic elements known in the art. Vectors include, for example, virus vectors and plasmid vectors.

Zscan4:

A group of genes that have previously identified as exhibiting 2-cell specific expression and ES cell-specific expression (PCT Publication No. WO 2008/118957) and have been shown to promote telomere elongation and genome stability (Zalzman et al., Nature 464(7290):858-863, 2010). In the context of the present disclosure, “Zscan4” includes both human ZSCAN4 and mouse Zscan4. In the mouse, the term “Zscan4” refers to a collection of genes including three pseudogenes (Zscan4-ps1, Zscan4-ps2 and Zscan4-ps3) and six expressed genes (Zscan4a, Zscan4b, Zscan4c, Zscan4d, Zscan4e and Zscan4f). Among the six paralogs, the open reading frames of Zscan4c, Zscan4d, and Zscan4f encode a SCAN domain as well as all four zinc finger domains, suggesting their potential role as transcription factors. Zscan4 refers to Zscan4 polypeptides and Zscan4 polynucleotides encoding the Zscan4 polypeptides. Exemplary sequences are provided herein (see SEQ ID NOs: 1-14).

Zscan4-ΔC:

In the context of the present disclosure, “Zscan4-ΔC” includes any mouse or human Zscan4 protein having a C-terminal deletion of at least one zinc finger domain. In some embodiments, the Zscan4-ΔC protein includes a deletion of at least two, such as three or all four zinc finger domains. SEQ ID NO: 2 and SEQ ID NO: 8 provide the amino acid sequences of human ZSCAN4 and mouse Zscan4c, respectively, and delineate the N-terminal SCAN domain and C-terminal zinc finger (C2H2-type) domains. In addition, the nucleotide and amino acid regions of each domain of human ZSCAN4 and mouse Zscan4c are listed below.

Human ZSCAN4

Nucleotides Amino Acids Domain (SEQ ID NO: 1) (SEQ ID NO: 2) SCAN  826-1074  44-126 C2H2-type 1 1630-1698 312-334 C2H2-type 2 1714-1782 340-362 C2H2-type 3 1798-1866 368-390 C2H2-type 4 1882-1950 396-418 Mouse Zscan4c

Nucleotides Amino Acids Domain (SEQ ID NO: 7) (SEQ ID NO: 8) SCAN 309-557  37-119 C2H2-type 1 1383-1451 395-417 C2H2-type 2 1470-1538 424-446 C2H2-type 3 1554-1622 452-474 C2H2-type 4 1638-1709 480-503

Zscan4-ERT2:

A fusion protein made up of a Zscan4 amino acid sequence and an ERT2 amino acid sequence. “Zscan4-ERT2” can also refer to a nucleic acid sequence encoding a Zscan4-ERT2 fusion protein. Exemplary amino acid sequences for Zscan4 (including SEQ ID NO: 2, 8, 10 and 14) and ERT2 (SEQ ID NO: 21) are set forth herein. In some embodiments, the Zscan4 sequence is a functional fragment or variant of a known Zscan4 sequence (such as SEQ ID NO: 2, 8, 10 or 14) and/or the ERT2 sequence is a functional fragment or variant of a known ERT2 sequence (such as SEQ ID NO: 21). Any fragment or variant of Zscan4 or ERT2 is contemplated as long as the fragment or variant retains activity. In some examples, the Zscan4-ERT2 fusion protein comprises a linker (or spacer) between Zscan4 and ERT2. Linkers are well known in the art and an appropriate linker can be selected by one of ordinary skill in the art. In particular examples, the linker is encoded by the nucleotide sequence GCTAGC.

Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The singular terms “a,” “an,” and “the” include plural referents unless context clearly indicates otherwise. Similarly, the word “or” is intended to include “and” unless the context clearly indicates otherwise. Hence “comprising A or B” means including A, or B, or A and B. It is further to be understood that all base sizes or amino acid sizes, and all molecular weight or molecular mass values, given for nucleic acids or polypeptides are approximate, and are provided for description. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including explanations of terms, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

III. Overview of Several Embodiments

The gold standard for examining the pluripotency of stem cells is to see whether cells can contribute to the entire body of an animal. It is disclosed herein that increasing the frequency of Zscan4 activation in mouse ES cells not only enhances, but also maintains their developmental potency in long-term cell culture. As the potency increases, even a whole animal can be produced from a single ES cell injected into a 4N blastocyst at an unexpectedly high success rate. Although Zscan4-activated cells express genes that are also expressed in 2-cell stage mouse embryos, the transiently Zscan4-activated state of ES cells is not associated with the high potency of ES cells. While not wishing to be bound by theory, these findings indicate that ES cells acquire higher potency by going through the transient Zscan4 activation state more frequently than the regular state. Taken together, these results demonstrate that the frequent activation of Zscan4 can rejuvenate pluripotent stem cells.

Particularly disclosed herein is the finding that the constitutive presence of Zscan4-ERT2, even in the absence of its usual activator tamoxifen, can increase the frequency of endogenous Zscan4 activation in ES cells, resulting in the increase of developmental potency of the ES cells. ES cells cultured in the accelerated Zscan4 activation cycle exhibited improved chimerism and potency, which are evidenced by a high contribution to chimeric mice and efficient production of a whole mouse from a single ES cell. Further disclosed herein is the finding that expression of C-terminally truncated Zscan4 (lacking the zinc finger domains) increases the number of Zscan4⁺ cells, thus having an effect similar to Zscan4-ERT2.

Accordingly, provided herein are isolated nucleic acid molecules encoding a Zscan4-ERT2 fusion protein. In particular examples, the Zscan4 is mouse Zscan4c or human ZSCAN4. Further provided are vectors comprising a Zscan4-ERT2 coding sequence, cells comprising such vectors (such as ES cells, iPS cells or other stem cells), and compositions that include the Zscan4-ERT2 encoding nucleic acid molecules or vectors. Further provided are recombinant Zscan4-ERT2 fusion proteins, cells comprising Zscan4-ERT2 fusion proteins and compositions that include the Zscan4-ERT2 fusion proteins.

Further provided herein are isolated nucleic acid molecules encoding a Zscan4ΔC protein (a Zscan4 protein having a deletion of at least one zinc finger domain). In particular examples, the Zscan4 is mouse Zscan4c or human ZSCAN4. Further provided are vectors comprising a Zscan4-ΔC coding sequence, cells comprising such vectors (such as ES cells, iPS cells or other stem cells), and compositions that include the Zscan4-ΔC encoding nucleic acid molecules or vectors. Further provided are recombinant Zscan4-ΔC proteins, cells comprising Zscan4-ΔC proteins and compositions that include the Zscan4-ΔC proteins.

Also provided herein are methods of using the Zscan4-ERT2 or Zscan4-ΔC nucleic acid molecules and proteins. For example, methods of enhancing or prolonging the pluripotency of a stem cell or a stem cell population by contacting the stem cell or stem cell population with a Zscan4-ERT2 nucleic acid molecule or fusion protein are disclosed herein. In other examples, methods of enhancing or prolonging the pluripotency of a stem cell or a stem cell population by contacting the stem cell or stem cell population with a Zscan4-ΔC nucleic acid molecule or protein are provided. Similarly, methods of increasing the frequency of Zscan4-positive cells in a stem cell population, as well as methods of promoting genome stability and/or increasing telomere length in a stem cell or a stem cell population, are provided.

A. Compositions, Vectors and Cells Comprising Zscan4-ERT2

Provided herein are isolated nucleic acid molecules encoding a fusion protein, wherein the fusion protein includes a Zscan4 protein fused to an ERT2 protein. ERT2 is a mutated version of the ligand binding domain of human estrogen receptor. ERT2 does not bind its natural ligand (17β-estradiol) at physiological concentrations, but is highly sensitive to nanomolar concentrations of tamoxifen or its metabolite 4-hydroxytamoxifen (4OHT).

In some embodiments, the nucleic acid molecule encoding the Zscan4-ERT2 fusion protein includes human ZSCAN4, mouse Zscan4c, mouse Zscan4d or mouse Zscan4f, or a functional fragment or variant thereof. Functional fragments and variants of Zscan4 include, for example, any Zscan4 fragment or variant that retains one or more biological activities of Zscan4, such as the capacity to increase pluripotency of a stem cell, promote genomic stability or increase telomere length.

Exemplary nucleic acid sequences for a variety of Zscan4 polynucleotides are known in the art (see, for example, PCT Publication No. WO 2008/118957) and are set forth herein, such as SEQ ID NO: 1 (human ZSCAN4), SEQ ID NO: 7 (mouse Zscan4c), SEQ ID NO: 9 (mouse Zscan4d) and SEQ ID NO: 13 (mouse Zscan4f). One skilled in the art will appreciate that sequences having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity to these sequences and retain Zscan4 activity are contemplated and can be used in the compositions and methods provided herein.

Zscan4 nucleic acid sequences from other species are also publically available, including dog ZSCAN4 (GenBank Accession Nos. XM_541370 and XM_848557); cow ZSCAN4 (GenBank Accession No. XM_001789250); and horse ZSCAN4 (GenBank Accession No. XM_001493944). Each of the above-listed GenBank Accession numbers is herein incorporated by references as it appears in the GenBank database on Feb. 22, 2011.

Fragments and variants of Zscan4 polynucleotides can readily be prepared by one of skill in the art using molecular techniques. In one embodiment, a fragment of a Zscan4 nucleic acid sequences includes at least 250, at least 500, at least 750, at least 1000, at least 1500, or at least 2000 consecutive nucleic acids of the Zscan4 polynucleotide. In a further embodiment, a fragment of Zscan4 is a fragment that confers a function of Zscan4 when expressed in a cell of interest, such as, but not limited to, promoting pluripotency, enhancing genome stability and/or increasing telomere length. The Zscan4 nucleic acid sequences contemplated herein include sequences that are degenerate as a result of the genetic code. There are 20 natural amino acids, most of which are specified by more than one codon. Therefore, all degenerate nucleotide sequences are included as long as the amino acid sequence of the Zscan4 polypeptide encoded by the nucleotide sequence is functionally unchanged.

In some embodiments, the Zscan4 nucleic acid sequence portion of the nucleic acid molecule encoding the Zscan4-ERT2 fusion protein is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 1, 7, 9 or 13. In some embodiments, the Zscan4 nucleic acid sequence comprises the nucleic acid sequence set forth in SEQ ID NO: 1, 7, 9 or 13. In some embodiments, the Zscan4 nucleic acid sequence consists of the nucleic acid sequence set forth in SEQ ID NO: 1, 7, 9 or 13.

In some examples, the Zscan4 portion of the Zscan4-ERT2 fusion protein comprises mouse Zscan4c. Thus, in particular examples, the Zscan4 nucleic acid sequence is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 7. In other examples, the Zscan4 comprises human ZSCAN4. In particular examples, the Zscan4 nucleic acid sequence is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 1.

In some embodiments, the nucleic acid sequence encoding the ERT2 portion of the Zscan4-ERT2 fusion protein is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to nucleotides 3989-4936 of SEQ ID NO: 19. In some examples, the nucleic acid sequence encoding ERT2 comprises or consists of nucleotides 3989-4936 of SEQ ID NO: 19.

In some embodiments, the nucleic acid molecule encoding the Zscan4-ERT2 fusion protein includes a linker sequence between the Zscan4 and ERT2 coding sequences. Linkers are well known in the art and selection of an appropriate linker is well within the capabilities of one of ordinary skill in the art. In some examples, the linker is at least 2 amino acids (aa), at least 3, at least 5, at least 10, at least 20, at least 50 or at least 100 aa, such as 2 to 50 or 2 to 10 aa. In particular examples disclosed herein, the linker includes the nucleic acid sequence GCTAGC (nucleotides 3983-3988 of SEQ ID NO: 19).

In some embodiments in which the Zscan4-ERT2 nucleic acid molecule encodes mouse Zscan4c, the nucleic acid molecule comprises a sequence at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to nucleotides 2465-4936 of SEQ ID NO: 19. In particular examples, the nucleic acid molecule comprises, or consists of, the sequence of nucleotides 2465-4936 of SEQ ID NO: 19.

In other embodiments in which the Zscan4-ERT2 nucleic acid molecule encodes human ZSCAN4, the nucleic acid molecule comprises a sequence at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to nucleotides 2479-4731 of SEQ ID NO: 20. In particular examples, the nucleic acid molecule comprises, or consists of, the sequence of nucleotides 2479-4731 of SEQ ID NO: 20. Also provided are vectors that include a Zscan4-ERT2 encoding nucleic acid molecule disclosed herein. Any suitable expression vector, such as an expression (plasmid) vector (e.g., pPyCAG-BstXI-IP), or viral vector (e.g., an adenovirus, adenoassociated virus, lentivirus or retrovirus vector), is contemplated. Numerous expression vectors and viral vectors are known in the art and the selection of an appropriate vector is well within the capabilities of one of ordinary skill in the art.

In some embodiments, the vector comprises a nucleotide sequence that is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 19 or SEQ ID NO: 20. In some examples, the vector comprises a nucleic acid sequence that is at least 95% identical to SEQ ID NO: 19 or SEQ ID NO: 20. In specific non-limiting embodiments, the nucleic acid sequence of the vector comprises, or consists of, SEQ ID NO: 19 or SEQ ID NO: 20.

Further provided herein are isolated cells containing a Zscan4-ERT2 nucleic acid molecule or vector as described herein. In some embodiments, the cell is a stem cell. In particular examples, the stem cell is an ES cell or an iPS cell. The origin of the stem cell can be from any suitable species. In some examples, the stem cell is a mouse, rat, human or non-human primate stem cell.

Compositions comprising a nucleic acid molecule or vector encoding a Zscan4ERT2 fusion protein are also provided herein. The compositions may further include a carrier or diluent, such as a pharmaceutically acceptable carrier or diluent. Zscan4-ERT2 fusion proteins encoded by the nucleic acid molecules and vectors described herein are further provided.

Also provided herein are recombinant Zscan4-ERT2 fusion proteins. In some embodiments, the recombinant Zscan4-ERT2 fusion protein includes human ZSCAN4, mouse Zscan4c, mouse Zscan4d or mouse Zscan4f, or a functional fragment or variant thereof. Functional fragments and variants of Zscan4 include, for example, any Zscan4 fragment or variant that retains one or more biological activities of Zscan4, such as the capacity to increase pluripotency of a stem cell, promote genomic stability or increase telomere length.

Exemplary amino acid sequences for a variety of Zscan4 proteins are known in the art (see, for example, PCT Publication No. WO 2008/118957) and are set forth herein, such as SEQ ID NO: 2 (human ZSCAN4), SEQ ID NO: 8 (mouse Zscan4c), SEQ ID NO: 10 (mouse Zscan4d) and SEQ ID NO: 14 (mouse Zscan4f). One skilled in the art will appreciate that sequences having at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity to these sequences and retain Zscan4 activity are contemplated and can be used in the methods provided herein.

Zscan4 amino acid sequences from other species are publically available, including dog ZSCAN4 (GenBank Accession Nos. XP_541370 and XP_853650); cow ZSCAN4 (GenBank Accession No. XP_001789302); and horse ZSCAN4 (GenBank Accession No. XP_001493994). Each of the above-listed GenBank Accession numbers is herein incorporated by references as it appears in the GenBank database on Feb. 22, 2011.

Fragments and variants of a Zscan4 protein can readily be prepared by one of skill in the art using molecular techniques. In one embodiment, a fragment of a Zscan4 protein includes at least 50, at least 100, at least 150, at least 200, at least 250, at least 300, at least 350, at least 400, at least 450 or at least 500 consecutive amino acids of the Zscan4 polypeptide. In a further embodiment, a fragment of Zscan4 is a fragment that confers a function of Zscan4, such as, but not limited to, increasing pluripotency, enhancing genome stability or increasing telomere length.

In some embodiments, the Zscan4 protein portion of the Zscan4-ERT2 fusion protein includes an amino acid sequence at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to the amino acid sequence set forth in SEQ ID NO: 2, 8, 10 or 14. In a further embodiment, the Zscan4 protein is a conservative variant of SEQ ID NO: 2, 8, 10 or 14, such that it includes no more than fifty conservative amino acid substitutions, such as no more than two, no more than five, no more than ten, no more than twenty, or no more than fifty conservative amino acid substitutions in SEQ ID NO: 2, 8, 10 or 14. In another embodiment, the Zscan4 peptide portion of the Zscan4-ERT2 fusion protein has an amino acid sequence comprising or consisting of the amino acid sequence set forth in SEQ ID NO: 2, 8, 10 or 14.

In some embodiments of the Zscan4-ERT2 fusion proteins, the Zscan4 comprises mouse Zscan4c. In some examples, the Zscan4c amino acid sequence is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to the amino acid sequence of SEQ ID NO: 8.

In other embodiments of the Zscan4-ERT2 fusion proteins, the Zscan4 portion comprises human ZSCAN4. In some examples, the ZSCAN4 amino acid sequence is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to the amino acid sequence of SEQ ID NO: 2.

In some embodiments, the amino acid sequence of the ERT2 portion of the Zscan4-ERT2 fusion protein is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 21. In some examples, the amino acid sequence of ERT2 comprises or consists of SEQ ID NO: 21.

In some embodiments, the Zscan4-ERT2 fusion protein includes a linker between the Zscan4 and ERT2 sequences. Linkers are well known in the art and selection of an appropriate linker is well within the capabilities of one of ordinary skill in the art. In particular examples disclosed herein, the linker includes the amino acid sequence Ala-Ser.

In some embodiments in which the Zscan4-ERT2 fusion protein includes mouse Zscan4c, the amino acid sequence of the fusion protein is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 22. In particular examples, the amino acid sequence of the Zscan4ERT2 fusion protein comprises, or consists of, the amino acid sequence of SEQ ID NO: 22.

In other embodiments in which the Zscan4-ERT2 fusion protein includes human ZSCAN4, the amino acid sequence of the fusion protein is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 23. In particular examples, the amino acid sequence of the Zscan4-ERT2 fusion protein comprises, or consists of, the amino acid sequence of SEQ ID NO: 23.

Further provided herein are isolated cells comprising a Zscan4-ERT2 fusion protein disclosed herein. In some embodiments, the cells are stem cells. In particular examples, the stem cells are ES cells or iPS cells. The origin of the stem cell can be from any suitable species. In some examples, the stem cell is a mouse, rat, human or non-human primate stem cell.

Compositions comprising a Zscan4-ERT2 fusion protein are also provided herein. The compositions may further include a carrier or diluent, such as a pharmaceutically acceptable carrier or diluent, for example saline.

B. Compositions, Vectors and Cells Comprising Zscan4-ΔC

Also provided herein are isolated nucleic acid molecules encoding a Zscan4 protein with a C-terminal truncation (referred to herein as Zscan4-ΔC). The C-terminally truncated Zscan4 comprises a deletion of at least one zinc finger domain. Thus, in some embodiments, the Zscan4-ΔC protein has a deletion of one, two, three or four zinc finger domains.

In some embodiments, the nucleic acid molecule encoding the Zscan4-ΔC protein includes C-terminally truncated human ZSCAN4, mouse Zscan4c, mouse Zscan4d or mouse Zscan4f. In particular embodiments, the Zscan4-ΔC protein is either human ZSCAN4 or mouse Zscan4c with a deletion of all four zinc finger domains. In one non-limiting example, the Zscan4-ΔC protein comprises the amino acid sequence of SEQ ID NO: 25 and/or is encoded by nucleotides 2465-3649 of SEQ ID NO: 24.

The Zscan4-ΔC nucleic acid sequences contemplated herein include sequences that are degenerate as a result of the genetic code. There are 20 natural amino acids, most of which are specified by more than one codon. Therefore, all degenerate nucleotide sequences are included as long as the amino acid sequence of the Zscan4-ΔC polypeptide encoded by the nucleotide sequence is functionally unchanged.

In some embodiments, the Zscan4-ΔC nucleic acid sequence is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to nucleotides 2465-3649 of SEQ ID NO: 24. In some embodiments, the Zscan4-ΔC nucleic acid sequence comprises the nucleic acid sequence set forth as nucleotides 2465-3649 of SEQ ID NO: 24. In some embodiments, the Zscan4-ΔC nucleic acid sequence consists of the nucleic acid sequence set forth as nucleotides 2465-3649 of SEQ ID NO: 24.

In some embodiments, the Zscan4-ΔC nucleic acid molecule is a human Zscan4-ΔC nucleic acid molecule. In particular examples, the human Zscan4-ΔC nucleic acid molecule comprises a deletion of at least nucleotides 1630-1950, nucleotides 1714-1950, nucleotides 1798-1950 or nucleotides 1882-1950 of SEQ ID NO: 1. In some embodiments, the human Zscan4-ΔC nucleic acid molecule is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to nucleotides 1-1629, nucleotides 1-1713, nucleotides 1-1797 or nucleotides 1-1881 of SEQ ID NO: 1. In some examples, the human Zscan4-ΔC nucleic acid molecule comprises or consists of nucleotides 1-1629, nucleotides 1-1713, nucleotides 1-1797 or nucleotides 1-1881 of SEQ ID NO: 1.

In some embodiments, the Zscan4-ΔC nucleic acid molecule is a mouse Zscan4ΔC nucleic acid molecule. In particular examples, the mouse Zscan4-ΔC nucleic acid molecule comprises a deletion of at least nucleotides 1383-1709, nucleotides 1470-1709, nucleotides 1554-1709 or nucleotides 1638-1709 of SEQ ID NO: 7. In some embodiments, the mouse Zscan4-ΔC nucleic acid molecule is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to nucleotides 1-1382, nucleotides 1-1469, nucleotides 1-1553 or nucleotides 1-1637 of SEQ ID NO: 7. In some examples, the mouse Zscan4-ΔC protein comprises or consists of nucleotides 1-1382, nucleotides 1-1469, nucleotides 1-1553 or nucleotides 1-1637 of SEQ ID NO: 7.

Also provided are vectors that include a Zscan4-ΔC encoding nucleic acid molecule disclosed herein. Any suitable expression vector, such as an expression (plasmid) vector (e.g., pPyCAG-BstXI-IP), or viral vector (e.g., an adenovirus, adeno-associated virus, lentivirus or retrovirus vector), is contemplated. Numerous expression vectors and viral vectors are known in the art and the selection of an appropriate vector is well within the capabilities of one of ordinary skill in the art.

In some embodiments, the vector comprises a nucleotide sequence that is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to SEQ ID NO: 24. In some examples, the vector comprises a nucleic acid sequence that is at least 95% identical to SEQ ID NO: 24. In specific non-limiting embodiments, the nucleic acid sequence of the vector comprises, or consists of, SEQ ID NO: 24.

Further provided herein are isolated cells containing a Zscan4-ΔC nucleic acid molecule or vector as described herein. In some embodiments, the cell is a stem cell. In particular examples, the stem cell is an ES cell or an iPS cell. The origin of the stem cell can be from any suitable species. In some examples, the stem cell is a mouse, rat, human or non-human primate stem cell.

Compositions comprising a nucleic acid molecule or vector encoding a Zscan4ΔC protein are also provided herein. The compositions may further include a carrier or diluent, such as a pharmaceutically acceptable carrier or diluent.

Zscan4-ΔC proteins encoded by the nucleic acid molecules and vectors described herein are further provided.

Also provided herein are recombinant Zscan4-ΔC proteins. In some embodiments, the recombinant Zscan4-ΔC protein includes C-terminally truncated human ZSCAN4, mouse Zscan4c, mouse Zscan4d or mouse Zscan4f.

In some embodiments, the Zscan4-ΔC protein includes an amino acid sequence at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to the amino acid sequence set forth in SEQ ID NO: 25. In a further embodiment, the Zscan4-ΔC protein is a conservative variant of SEQ ID NO: 25, such that it includes no more than fifty conservative amino acid substitutions, such as no more than two, no more than five, no more than ten, no more than twenty, or no more than fifty conservative amino acid substitutions in SEQ ID NO: 25. In another embodiment, the Zscan4-ΔC protein has an amino acid sequence comprising or consisting of the amino acid sequence set forth in SEQ ID NO: 25.

In some embodiments, the Zscan4-ΔC protein is a human Zscan4-ΔC protein. In particular examples, the human Zscan4-ΔC protein comprises a deletion of at least amino acids 312-418, amino acids 340-418, amino acids 368-390 or amino acids 396418 of SEQ ID NO: 2. In some embodiments, the human Zscan4-ΔC protein is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to amino acids 1-311, amino acids 1-339, amino acids 1-367 or amino acids 1-395 of SEQ ID NO: 2. In some examples, the human Zscan4-ΔC protein comprises or consists of amino acids 1-311, amino acids 1-339, amino acids 1-367 or amino acids 1-395 of SEQ ID NO: 2.

In some embodiments, the Zscan4-ΔC protein is a mouse Zscan4-ΔC protein. In particular examples, the mouse Zscan4-ΔC protein comprises a deletion of at least amino acids 395-503, amino acids 424-503, amino acids 452-503 or amino acids 480-503 of SEQ ID NO: 8. In some embodiments, the mouse Zscan4-ΔC protein is at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% identical to amino acids 1-394, amino acids 1-423, amino acids 1-451 or amino acids 1-479 of SEQ ID NO: 8. In some examples, the mouse Zscan4-ΔC protein comprises or consists of amino acids 1-394, amino acids 1-423, amino acids 1-451 or amino acids 1-479 of SEQ ID NO: 8.

Further provided herein are isolated cells comprising a Zscan4-ΔC protein disclosed herein. In some embodiments, the cells are stem cells. In particular examples, the stem cells are ES cells or iPS cells. The origin of the stem cell can be from any suitable species. In some examples, the stem cell is a mouse, rat, human or non-human primate stem cell.

Compositions comprising a Zscan4-ΔC protein are also provided herein. The compositions may further include a carrier or diluent, such as a pharmaceutically acceptable carrier or diluent, for example saline.

C. Recurrent Activation of Zscan4 in Stem Cells and Methods of Use

Disclosed herein is the finding that recurrent activation of Zscan4 enhances the pluripotency of stem cells. In particular, it is disclosed herein that increasing the frequency of Zscan4 activation in ES cells enhances and maintains developmental potency in long-term culture. The results described in the Examples below indicate that ES cells acquire higher potency by going through the transient Zscan4 activation state more frequently than the regular state.

Thus, provided herein are methods of enhancing or prolonging the pluripotency of a stem cell or a stem cell population by inducing frequent activation of Zscan4 in the stem cell or stem cell population. Methods of increasing the frequency of Zscan4-positive cells in a stem cell population by inducing frequent activation of Zscan4 are also provided. Further provided are methods of promoting genome stability or increasing telomere length, or both, in a stem cell or a stem cell population by promoting recurrent activation of Zscan4 in the stem cell or stem cell population.

In some embodiments of the methods disclosed herein, the methods include contacting the stem cell or stem cell population with (i) a nucleic acid molecule encoding a Zscan4-ERT2 fusion protein or a composition thereof, (ii) a vector encoding a Zscan4-ERT2 fusion protein or a composition thereof, or (iii) a Zscan4-ERT2 fusion protein or a composition thereof.

In other embodiments of the methods disclosed herein, the methods include contacting the stem cell or stem cell population with (i) a nucleic acid molecule encoding a Zscan4-ΔC protein or a composition thereof, (ii) a vector encoding a Zscan4-ΔC protein or a composition thereof, or (iii) a Zscan4-ΔC protein or a composition thereof.

In other embodiments, a stem cell or stem cell population is contacted with an agent that promotes frequent activation of Zscan4. The agent can be, for example, any nucleic acid molecule, polypeptide, small molecule or other compound that results in recurrent activation of Zscan4 in a cell.

In some examples, the stem cell is an ES cell or an iPS. The methods can be applied to stem cells of any species, for example, mouse, rat, human or non-human primate stem cells.

1. Enhancing or Prolonging Pluripotency of Stem Cells

Provided herein is a method of enhancing or prolonging the pluripotency of a stem cell or a stem cell population. In some embodiments, the method includes contacting the stem cell or stem cell population with a nucleic acid molecule or vector encoding a Zscan4-ERT2 fusion protein as disclosed herein. In other embodiments, the method includes contacting the stem cell or stem cell population with a Zscan4-ERT fusion protein disclosed herein.

In yet other embodiments, the method includes contacting the stem cell or stem cell population with a nucleic acid molecule or vector encoding a Zscan4-ΔC protein as disclosed herein. In other embodiments, the method includes contacting the stem cell or stem cell population with a Zscan4-ΔC protein disclosed herein.

Methods of delivering a nucleic acid molecule into a cell are well known in the art. In some examples, “contacting” the stem cell with a nucleic acid molecule or vector includes transfection (such as liposomal-mediated transfection), electroporation, injection or any other suitable technique for introducing a nucleic acid molecule into a cell.

Methods for delivery of proteins to cells are also well known in the art. In some examples, the Zscan4-ERT2 fusion protein or Zscan4-ΔC protein is encapsulated by a nanoparticle to aid in delivery to the cells. Suitable nanoparticles for use with the disclosed methods are known in the art and are described briefly below.

The nanoparticles for use with the methods described herein can be any type of biocompatible nanoparticle, such as biodegradable nanoparticles, such as polymeric nanoparticles, including, but not limited to polyamide, polycarbonate, polyalkene, polyvinyl ethers, and cellulose ether nanoparticles. In some embodiments, the nanoparticles are made of biocompatible and biodegradable materials. In some embodiments, the nanoparticles include, but are not limited to nanoparticles comprising poly(lactic acid) or poly(glycolic acid), or both poly(lactic acid) and poly(glycolic acid). In particular embodiments, the nanoparticles are poly(D,L-lactic-co-glycolic acid) (PLGA) nanoparticles.

Other biodegradable polymeric materials are contemplated for use with the methods described herein, such as poly(lactic acid) (PLA) and polyglycolide (PGA). Additional useful nanoparticles include biodegradable poly(alkylcyanoacrylate) nanoparticles (Vauthier et al., Adv. Drug Del. Rev. 55: 519-48, 2003).

Various types of biodegradable and biocompatible nanoparticles, methods of making such nanoparticles, including PLGA nanoparticles, and methods of encapsulating a variety of synthetic compounds, proteins and nucleic acids, has been well described in the art (see, for example, U.S. Publication No. 2007/0148074; U.S. Publication No. 20070092575; U.S. Patent Publication No. 2006/0246139; U.S. Pat. Nos. 5,753,234; 7,081,489; and PCT Publication No. WO/2006/052285).

Methods of assessing the pluripotency of a cell are known in the art. Example 2 below describes exemplary methods that can be used to evaluate the potency of an ES cell. In one example, ES cells are injected into mouse blastocysts, transferred to uteri and the extent of ES cell potency is determined by the percent chimerism of the pups based on coat color. In another example, a 4N complementation assay is performed. In this assay, ES cells are injected into a tetraploid (4N) blastocyst. Potency of the ES cells is determined by the ability of the ES cells to produce live embryos.

In some examples, the pluripotency of a stem cell or a stem cell population is increased by at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or at least 100%, as compared to the pluripotency of a stem cell or a stem cell population in the absence of increased Zscan4 activation frequency (such as in the absence of expression of an Zscan4-ERT2 fusion protein).

Also provided herein is a method for increasing the developmental potency of a stem cell or a stem cell population by transiently overexpressing Zscan4. In one embodiment, the overexpressed Zscan4 is mouse Zscan4c.

Further provided herein is a method of enhancing or prolonging the pluripotency of a stem cell or a stem cell population, by contacting the stem cell or stem cell population with an isolated nucleic acid molecule encoding a Zscan4 protein or a vector that includes a nucleic acid molecule encoding a Zscan 4 protein. In an embodiment employing a vector, the vector includes an inducible promoter.

2. Increasing the Frequency of Zscan4⁺ Cells in a Population

Also provided herein is a method of increasing the frequency of Zscan4-positive cells in a stem cell population. In some embodiments, the method includes contacting the stem cell population with a nucleic acid molecule or vector encoding a Zscan4-ERT2 fusion protein disclosed herein.

In other embodiments, the method includes contacting the stem cell population with a Zscan4-ERT fusion protein disclosed herein. In yet other embodiments, the method includes contacting the stem cell population with a nucleic acid molecule or vector encoding a Zscan4-ΔC protein disclosed herein. In other embodiments, the method includes contacting the stem cell population with a Zscan4-ΔC protein disclosed herein.

Methods of delivering nucleic acid molecules encoding Zscan4-ERT2 or Zscan4-ΔC, and Zscan4-ERT2 or Zscan4-ΔC proteins to stem cells are known in the art and are described above.

Methods of detecting Zscan4⁺ cells in a cell population are routine and have been previously described (see for example, PCT Publication No. WO 2008/118957, herein incorporated by reference). For example, antibodies specific for Zscan4 (which are commercially available or can be produced according to standard procedures) can be used in immunological based assays to detect Zscan4⁺ cells. For instance, fluorescence-activated cell sorting can be used to detect and quantify Zscan4⁺ cells in a population. As another example, a Zscan4 reporter construct can be used to detect expression of Zscan4 (such as the pZscan4-Emerald vector as described in PCT Publication No. WO 2008/118957).

In particular examples, the increase in frequency of Zscan4⁺ cells in the population is an increase of at least 5%, at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 40%, at least 50%, at least 50%, at least 75%, at least 90% or at least 100%. The increase is relative to, for example, a population of cells that has not been contacted with a Zscan4-ERT2 nucleic acid or fusion protein, or a Zscan4-ΔC nucleic acid or protein (and thus has not undergone frequent activation of Zscan4).

3. Promoting Genome Stability and Increasing Telomere Length

Methods of promoting genome stability or increasing telomere length, or both, in a stem cell or a stem cell population are further provided. In some embodiments, the method includes contacting the stem cell or stem cell population with a nucleic acid molecule or vector encoding a Zscan4-ERT2 fusion protein disclosed herein. In other embodiments, the method includes contacting the stem cell or stem cell population with a Zscan4-ERT fusion protein disclosed herein.

In yet other embodiments, the method includes contacting the stem cell or stem cell population with a nucleic acid molecule or vector encoding a Zscan4-ΔC protein disclosed herein. In other embodiments, the method includes contacting the stem cell or stem cell population with a Zscan4-ΔC protein disclosed herein.

Methods of delivering nucleic acid molecules encoding Zscan4-ERT2 or Zscan4-ΔC, and Zscan4-ERT2 or Zscan4-ΔC proteins to stem cells are known in the art and are described above.

In particular examples, genome stability is increased in a stem cell by at least 20%, at least 40%, at least 50%, at least 60%, at least 75%, at least 80%, at least 90%, at least 95%, or at least 98%, for example relative to stem cell that has not been contacted with a Zscan4-ERT2 or Zscan4-ΔC protein or a nucleic acid encoding a Zscan4-ERT2 or Zscan4-ΔC protein (or compared to a value or range of values expected in a stem cell that has not undergone frequent activation of Zscan4). Methods of measuring genome stability and telomere length are routine in the art, and the disclosure is not limited to particular methods. The particular examples provided herein are exemplary.

In some examples, genome stability in a stem cell is measured by detecting cell proliferation. Genome stability is increased if cell proliferation is increased, for example relative to a control cell (for example, a stem cell that has not been contacted with a Zscan4-ERT or Zscan4-ΔC protein or nucleic acid). For example, ES cell proliferation can be detected by growing ES cells in culture and measuring the doubling time of the cells after each passage. In one example, genome stability is increased if crisis (e.g., cell death) does not occur at passage 8 or earlier.

In some examples, genome stability in a stem cell, such as an ES cell or iPS cells, is measured by performing karyotype analysis. Genome stability is increased if the presence of karyotype abnormalities (such as chromosome fusions and fragmentations) is decreased or even absent, for example relative to a cell that has not undergone frequent activation of Zscan4. For example, karyotype analysis can be performed in stem cells by inducing metaphase arrests, then preparing metaphase chromosome spreads.

In some examples, genome stability in stem cell is measured by measuring telomere sister chromatid exchange (T-SCE). Genome stability is increased if the presence of T-SCE is increased relative to a control (such as a stem cell that has not undergone frequent activation of Zscan4). For example, T-SCE can be measured in an stem cell by using telomere chromosome-orientation FISH (CO-FISH).

In some examples, genome stability in stem cell is measured by measuring sister chromatid exchange (SCE). Genome stability is increased if the presence of SCE is decreased relative to a control, such as a stem cell that has not undergone frequent activation of Zscan4. For example, SCE can be measured in a stem cell by detecting SCE in a metaphase spread.

In some examples, telomere length is measured in stem cell. Telomere length is increased in a stem cell if the length of the telomeres is greater, for example relative to telomere length in a control cell that has not undergone frequent activation of Zscan4 (such as a cell that has not been contacted with a Zscan4-ERT2 or Zscan4-ΔC protein or nucleic acid). For example, telomere length can be detected in a stem cell by fluorescence in situ hybridization (FISH), quantitative FISH (Q-FISH), or telomere qPCR.

The following examples are provided to illustrate certain particular features and/or embodiments. These examples should not be construed to limit the disclosure to the particular features or embodiments described.

EXAMPLES Example 1 Materials and Methods

This example describes the experimental procedures used for the studies described in Example 2.

ES Cell Culture

MC1 ES cells derived from 12956/SvEvTac and MC2 ES cells derived from C57BL/6J (Olson et al., Cancer Res 63:6602-6606, 2003) were purchased from the Transgenic Core Laboratory of the Johns Hopkins University School of Medicine (Baltimore, Md.). V6.5 ES cells (Eggan et al., Proc Natl Acad Sci USA 98:6209-6214, 2001) derived from an F1 hybrid strain (C57BL/6×129/Sv) were purchased from Thermo Scientific Open Biosystem. All ES cell lines, except for TA1 ES cell line (see below), were cultured at 37° C. in 5% CO² in the complete ES medium as previously described (Zalzman et al., Nature 464:858-863, 2010): DMEM (Gibco), 15% FBS (Atlanta Biologicals), 1000 U/ml leukemia inhibitory factor (LIF) (ESGRO, Chemicon), 1 mM sodium pyruvate, 0.1 mM non-essential amino acids (NEAA), 2 mM GlutaMAX™, 0.1 mM beta-mercaptoethanol, and penicillin/streptomycin (50 U/50 μg/ml). TA1 ES cell lines were cultured as described above. For all cell lines, media was changed daily and cells were passaged every 2 to 3 days routinely.

Derivation of TA1 ES Cell Line

C57BL/6J females (The Jackson Laboratory, Bar Harbor, Me.) and 12956/SvEvTac males (Taconic) were naturally mated to collect 2-cell embryos, which were then cultured in KSOM medium for 3 days at 37° C. in 5% CO². Resulting blastocysts were transferred onto mouse embryo fibroblast (MEF) feeder cells treated with mitomycin C (Sigma) and cultured for 7 days in the complete ES medium (described above) after replacing 15% FBS with 15% KSR (Invitrogen) and adding 50 nM PD98059 (MEK1 inhibitor). After picking inner cell mass (ICM) clumps and dissociating them by ACCUTASE™ (Millipore), they were seeded onto fresh feeder cells and cultured in the same condition for an additional 7 days. Newly derived ES cell lines were directly tested for their developmental potency by 4N-complementation (see below).

pCAG-Zscan4-ERT2 Vector Construction

Genes collectively called Zscan4 consist of 6 paralogous genes and 3 pseudogenes clustered on a ˜850 kb region of chromosome 7 (Falco et al., Dev Biol 307:539-550, 2007). Among six paralogs named Zscan4a to Zscan4f, the open reading frames (ORFs) of Zscan4c, Zscan4d, and Zscan4f are very similar to each other and encode a SCAN domain and four zinc finger domains (Falco et al., Dev Biol 307:539550, 2007). To construct a pCAG-Zscan4-ERT2 plasmid, an entire ORF (506 a.a.) of mouse Zscan4c gene (Falco et al., Dev Biol 307:539-550, 2007) was fused with ERT2 (Feil et al., Proc Natl Acad Sci USA 93:10887-10890, 1996) (314 a.a.) and cloned into XhoI/NotI sites of pPyCAG-BstXI-IP (Niwa et al., Gene 108:193-199, 1991). The resultant plasmid vector expresses Zscan4c-ERT2 fusion protein-IRES-puromycin-resistant protein under a strong CAG promoter.

Generation of ZE and ZERT2 ES Cell Clones

ES cells were grown in 6-well plates. For ZE ES cell clones, 5×10⁵ ES cells in suspension were transfected with 1 μg of a linearized pZscan4-Emerald vector (Zalzman et al., Nature 464:858-863, 2010) using EFFECTENE™ (QIAGEN) according to manufacturer's protocol, and plated in 100 mm dishes. After selecting with 5 μg/ml blasticidin for 8 days, resulting ES cell colonies were picked, expanded, and frozen for further analysis. For ZERT2 ES cell clones, 5×10⁵ ES cells in suspension were cotransfected with 0.5 μg of a linearized pCAG-Zscan4-ERT2 vector and 0.5 μg of PL452 (PGK promoter-Neo) (Liu et al., Genome Res 13:476-484, 2003) using EFFECTENE™ (QIAGEN) according to manufacturer's protocol, and plated in 100 mm dishes. After selecting with G418 for 8 days, resulting ES cell colonies were picked, expanded, and frozen for further analysis.

Quantitative Reverse Transcription Polymerase Chain Reaction (qRT-PCR)

RNA was isolated from cells by TRIZOL™ (Invitrogen) in biological triplicate. One μg of total RNA was reverse transcribed by SuperScript™ III (Invitrogen) following the manufacturer's protocol. 100 ng of oligo dT primers (Promega) was used per reaction. For qPCR, SYBR™ green master mix (Applied Biosystems) was used following the manufacturer's protocol. 96-well optical plates with a 25 μl total reaction volume were used, 10 ng of cDNA was used per well. Plates were run on 7300 or 7500 system (Applied Biosystems). Fold induction was calculated by the ΔΔCt method (Livak et al., Methods 25:402-408, 2001) using H2A as normalizer.

RNA Isolation, cDNA Preparation and qPCR Analysis in Mouse Preimplantation Embryos

Four to six week-old B6D2F1 female mice were superovulated with 5 I.U. of PMSG (Sigma) and 5 I.U. of human chorionic gonadotropin (hCG) (Sigma). Eggs or embryos for qRT-PCR experiments were collected after 20, 23, 30, 43, 55, 66, 80 and 102 hours post hCG injection for MII (unfertilized oocytes), 1-cell, early and late-2 cell, 4-cell, 8-cell, morula and blastocyst embryos, respectively. Three sets of 10 synchronized eggs or embryos were stored in liquid nitrogen and mechanically ruptured by a freeze/thaw step for the cDNA preparation template. Oligo-dT primers and SuperScript™ III reverse transcriptase (Invitrogen) were used according to the manufacturer's instruction. Analysis was performed on the ABI 7300 Fast Real Time PCR system (Applied Biosystems). Data was normalized by Chuk (Falco et al., Reprod Biomed Online 13:394-403, 2006) with the ΔΔCt method (Livak et al., Methods 25:402-408, 2001).

RNA In Situ Hybridization

Whole mount in situ hybridization was performed as previously described (Carter et al., Gene Expr Patterns 8:181-198, 2008). Briefly, ES cells in triplicates, grown for 3 days, were fixed in 4% paraformaldehyde (PFA) at 4° C. overnight. After digestion with proteinase K, cells were hybridized with 1 μg/ml digoxigenin-labeled riboprobe at 62° C. overnight. Cells were then washed, blocked, incubated with alkaline phosphatase-conjugated anti-digoxigenin antibody, and incubated with NBT/BCIP detection buffer for 30 minutes or overnight.

Double-Fluorescence RNA In Situ Hybridization

Digoxigenin (DIG)- and biotin (BIO)-labeled RNA probes were transcribed from the PCR product templates using RNA Labeling Mix (Roche). Ethanol-precipitated probes were resuspended in water and quantified by RNA 6000 Nano Assay on a 2100 Bioanalyzer (Agilent Technologies). 10⁵ cells/well were seeded in glass chamber slides, cultured for three days, fixed with PFA, and permeabilized with 0.5% TritonX-100. Cells were washed and incubated with 1 μg/ml DIG and BIO probes for 12 hours at 60° C. in hybridization solution. Probes were detected by mouse anti-DIG antibody and by sheep anti-BIO, and visualized by fluorophore-conjugated secondary antibodies. Nuclei were stained with DAPI (blue).

Microarray Analysis

DNA microarray analyses were carried out as described (Aiba et al., DNA Res 16:73-80, 2009). Briefly, universal Mouse Reference RNA (Stratagene) were labeled with Cy5-dye, mixed with Cy3-labeled samples, and used for hybridization on the NIA Mouse 44K Microarray v2.2 (Carter et al., Genome Biol 6:R61, 2005) (manufactured by Agilent Technologies #014117). The intensity of each gene feature was extracted from scanned microarray images using Feature Extraction 9.5.1.1 software (Agilent Technologies). Microarray data analyses were carried out by using an application developed in-house to perform ANOVA and other analyses (NIA Array Analysis software; online at lgsun.grc.nia.nih.gov/ANOVA/) (Sharov et al., Bioinformatics 21:2548-2549, 2005). All the DNA microarray data have been deposited to the NCBI Gene Expression Omnibus (GEO, online at www.ncbi.nlm.nih.gov/geo/) and are accessible through GEO Series accession number (GSE26278) and the NIA Array Analysis software website (online at lgsun.grc.nia.nih.gov/ANOVA/) (Sharov et al., Bioinformatics 21:2548-2549, 2005). For GEO reviewer link: www.ncbi.nlm.nih.gov/geo/query/acc.cgi?token=fhaxtmiueykigvm&acc=GSE26278.

ES Cell Injection into 2N or Tetraploid (4N) Blastocysts

CD1 females (Charles River, 8-12 week old) were used for superovulation by PMSG (Sigma) followed 48 hours later by hCG (Sigma) administration. After hCG administration, females were mated with males of the same strain and 2-cell embryos were collected by flushing oviducts. Recovered embryos were cultured in KSOM (Millipore) medium for 3 days at 37° C. in 5% CO². Collected 2-cell embryos were directly transferred into 0.3 M mannitol solution and aligned automatically by alternate current (AC) pulse in an electrofusion chamber. Then two direct current (DC) pulses with 140V/mm were applied for 40 μs using LF101 Electro Cell Fusion Generator. Fused embryos (4N) that had one blastomere were collected at 60 minutes cultivation and then culture continued in KSOM medium until they reached the blastocyst stage. A single ES cell or 10-15 ES cells were injected into 2N or 4N blastocysts to assess their developmental potency and then transferred to E2.5 recipient females. To study the effects of Tmx on ES cells, ES cells were cultured in the presence of 200 nM Tmx for 2-3 days before injection.

Example 2 Rejuvenation of Pluripotent Stem Cells by Frequent Activation of Zscan4

This example describes the finding that increasing the frequency of Zscan4 activation in mouse ES cells not only enhances, but also maintains their developmental potency in long-term cell culture.

Commonality Between Transient Zscan4⁺ State and 2-Cell Stage Embryos

As a first step to characterize the Zscan4⁺ state of ES cells, global gene expression profiles were compared between Zscan4⁺ and Zscan4⁺ state of ES cells. In an earlier study, a reporter cell line, pZscan4-Emerald cells (hereafter called “MC1ZE”), was established in which a Zscan4c-promoter-driven reporter green fluorescence protein GFP-Emerald (Em) recapitulates the expression of endogenous Zscan4 (Zalzman et al., Nature 464:858-863, 2010). DNA microarray analysis of FACS-sorted Em⁺ and Em⁻ cells was carried out. Em⁺ cells showed a very similar gene expression profile to the Em⁻ cells with only 161 differentially expressed genes (FIG. 5; see also PCT Publication No. WO 2008/118957 and Falco et al., Dev Biol 307:539-550, 2007). Pluripotency-related markers remained unchanged in Em⁺ cells compared to Em⁻ cells, but Tcstv1 and Tcstv3 (two cell-specific transcript variant 1 and 3) genes (Struwe and Solter, GenBank accession AF067057.1; Zhang et al., Nucleic Acids Res 34:4780-4790, 2006) were among the most highly upregulated genes (FIG. 5). RNA whole-mount in situ hybridization revealed “Zscan4-like” expression for 7 other genes in the list (Tcstv1/3, Eif1a, Pif1, AF067063, EG668777, RP23-149D11.5, BC061212, and EG627488; see PCT Publication No. WO 2008/118957, herein incorporated by reference).

Furthermore, double-label fluorescence RNA in situ hybridization confirmed co-expression of these genes with Zscan4. As Zscan4 is a 2-cell embryo marker (Falco et al., Dev Biol 307:539-550, 2007), 6 genes were selected from the list based on additional gene expression information in preimplantation embryos (Ko et al., Development 127:1737-1749, 2000; Sharov et al., PLoS Biol 1:E74, 2003) and were examined for their expression profiles by qRT-PCR. All six genes tested showed a high expression peak in 2-cell embryos: 2 genes showed the highest peak at the late 2-cell stage as Zscan4, whereas 4 others showed their highest peak at the early 2-cell stage (see PCT Publication No. WO 2008/118957, herein incorporated by reference). Considering the fact that a large-scale screening of ˜250 transcription factor genes by whole-mount in situ hybridization identified only two other genes (Rhox9 and Whsc2) with a “Zscan4-like” expression pattern (Carter et al., Gene Expr Patterns 8:181-198, 2008), the high incidence of finding 2-cell genes with a Zscan4-like expression pattern in ES cells suggests that some of the gene expression program in early-stage embryos are reactivated in the Zscan4 state of ES cells.

Transient Zscan4⁺ State is not Associated with Higher Developmental Potential

ES cells are thought to be equivalent to cells in the inner cell mass (ICM) of blastocysts (Nichols and Smith, Development 138:3-8, 2011; Yoshikawa et al., Gene Expr Patterns 6:213-224, 2006). Commonality between Zscan4⁺ state and 2-cell embryos suggest that in standard cell culture conditions, ES cells are a mixed population of ˜5% of 2-cell like cells and ˜95% of ICM-like cells. As it has been shown that by nuclear transplantation (cloning) the 2-cell nucleus has a higher developmental potential than the ICM nucleus (Tsunoda et al., Development 107:407-411, 1989; Kono et al., J Reprod Fertil 93:165-172, 1991), the Zscan4⁺ state may represent high-potential true stem cells among the regular ES cell population.

To test this notion, V6.5 ZE cells (clone #17) were generated and their developmental potency was assessed by transfecting a pZscan4-Emerald vector into V6.5 ES cells derived from an F1 hybrid strain (C57BL/6×129/Sv), which has been extensively used for testing developmental potency (Eggan et al., Proc Natl Acad Sci USA 98:6209-6214, 2001; Wang and Jaenisch, Dev Biol 275:192-201, 2004). To avoid cell damage caused by cell sorting or long UV exposure, Em⁺ or Em⁻ cells were separated manually by pipetting, single ES cells were injected into 2N blastocysts, and the subsequent embryo development was observed. Based on the coat colors, it was found that Em⁻ ES cells were able to contribute to the tissues of chimeric mice at a relatively high rate (31%), whereas Em⁺ ES cells were not (0%). The results indicate that, contrary to expectations, Zscan4⁺ cells are not associated with high developmental potency compared to Zscan4⁻ cells.

Zscan4-ERT2 Increases the Frequency of Endogenous Zscan4⁺ Cells in the Absence of Tmx

Intermittent and transient activation of Zscan4 is required for the long-term maintenance of ES cell cultures (Zalzman et al., Nature 464:858-863, 2010). It was therefore hypothesized that more frequent activation of Zscan4 further improves the quality of ES cells, including their developmental potency. A system to mimic the transient expression of Zscan4 was sought. To this end, ERT2, the tamoxifen (Tmx) inducible system was selected (Feil et al., Proc Natl Acad Sci USA 93:10887-10890, 1996). This system allows one to keep a transgene off in the absence of Tmx and turn it on in the presence of Tmx at will (Feil et al., Proc Natl Acad Sci USA 93:10887-10890, 1996). First, the plasmid construct pCAG-Zscan4-ERT2 was made in which Zscan4c open reading frame (ORF) fused with ERT2 domain can be driven by a strong ubiquitous promoter CAG (Niwa et al., Gene 108:193-199, 1991) (FIG. 1A).

When the pCAG-Zscan4-ERT2 plasmid was transfected into MC1-ZE3 cells, it was found that the constitutive expression of Zscan4-ERT2 in ES cells increased the fraction of Em⁺ cells even in the Tmx⁻ condition (FIG. 1B). Adding Tmx to the culture media further increased the fraction of Em⁺ cells, but also made ES cells (both Em⁺ and Em⁻ cells) flatter, resulting in the flattening of ES cell colonies—a deviation from the typical pluripotent ES colony morphologies (FIG. 1B). The results were further confirmed by quantitative assays for five independent clones: the constitutive expression of Zscan4-ERTs even in the absence of Tmx caused a 3-fold increase of Em⁺ cells by the flow cytometry analysis (FIG. 1C) and 5-fold increase by the qRT-PCR analysis (FIG. 1D); and addition of Tmx to the medium caused further 2-fold and 1.2 fold increase, respectively (FIGS. 1C-1D).

To further investigate this unexpected result, the pCAG-Zscan4-ERT2 plasmid was transfected into V6.5 ES cells (Eggan et al., Proc Natl Acad Sci USA 98:6209-6214, 2001) and multiple cell clones named V6.5 ZERT2 were isolated. Based on the qRT-PCR analysis of Zscan4 ORF, clone #18 was selected for the highest Zscan4 expression levels, clones #7 and #10 were selected for the second and third highest Zscan4 levels, and clone #2 was selected with the background Zscan4 level (FIG. 6A). Based on genotyping by PCR, clone #2 did not have any copies of the pCAG-Zscan4ERT2 plasmid, and was thus used as a control (V6.5 #2). As expected, Tmx⁺ conditions slowed down the proliferation of ES cells (FIG. 6B) and made ES cells flatter (FIG. 6C). When the Tmx was removed from the medium after 10 passages in the Tmx⁺ conditions, the cell proliferation and morphology returned to normal (FIGS. 6B-6C), suggesting that effects of Tmx on the V6.5 ZERT2 cells were reversible.

To check if the frequency of Zscan4⁺ cells is increased even in the Tmx condition, whole mount in situ hybridization was carried out using a full-length Zscan4c probe to detect both endogenous and exogenous copies of Zscan4 as well as a 3′-UTR Zscan4c probe to detect only endogenous Zscan4. The results showed ˜3-fold increase of the number of Zscan4⁺ cells in V6.5 ZERT2 ES cell clones (#7, #10, and #18) in the absence of Tmx compared to the usual level of Zscan4⁺ cells in the control cells (V6.5 and V6.5 #2) (FIG. 1E). Further comparison of global gene expression profiles by DNA microarrays confirmed that the expression of Zscan4 was upregulated by 3.6-fold in V6.5 ZERT2 #18 ES cells even in the Tmx⁻ condition (FIGS. 7 and 8). Similarly, other key Zscan4-related genes identified in Falco et al. (Dev Biol 307:539-550, 2007), such as Tcstv1, Tcstv3, Tmem92, RP23-149D11.5, and BC061212, were also upregulated in V6.5 ZERT2 #18 ES cells in the Tmx⁻ condition (FIG. 1F, FIG. 7, FIG. 8). Adding Tmx increased the expression of Zscan4 and other Zscan4-related genes only slightly, but increased that of Zscan4-unrelated genes significantly (FIG. 1G, FIG. 7 and FIG. 9). Taken together, use of constitutively expressing Zscan4-ERT2 without Tmx became an unexpected, but attractive strategy to enhance the naturally occurring Zscan4 effects by increasing the number of endogenous Zscan4⁺ cells.

Zscan4 Protein Lacking the C-Terminus (Zscan4c-ΔC) Increases the Number of Zscan4⁺ Cells

Based on the results described above, it was hypothesized that the effect of ERT2 was due to blocking the function of the Zscan4 zinc finger domains at the C-terminus of the protein. Thus, to evaluate whether C-terminally truncated Zscan4 has the same effect as Zscan4-ERT2 of inducing recurrent activation of Zscan4, vectors encoding either C-terminal truncated (lacking all four zinc finger domains) or N-terminal truncated (lacking the SCAN domain) Zscan4 were constructed. FIG. 2A provides a schematic of the structure of Zscan4c, Zscan4c-ERT2, Zscan4c-ΔC and Zscan4c-ΔN proteins. The amino acid sequence of Zscan4c-ΔC is set forth herein as SEQ ID NO: 25.

The mutated Zscan4c genes were placed under the strong and constitutive CAG promoter. The sequence of the pCAG-Zscan4-ΔC vector is set forth herein as SEQ ID NO: 24. Each vector was transfected into MC1-ZE16 ES cells (sister clones of MC1-ZE3). Multiple independent clones were isolated: ZDC-MC1-ZE16 #3, #4, #20 for Zscan4c-ΔC; ZDN-MC1-ZE16 #5, #8, #15 for Zscan4c-ΔN. Fluorescence microscopy was performed on each cell clone. The images of ZDC-MC1-ZE16 #3, #4, #20 and ZDN-MC1-ZE16 #5, #8, #15 are shown in FIGS. 2B-2G. The results clearly show that the expression of Zscan4c-ΔC increases the number of Zscan4⁺ cells, whereas the expression of Zscan4c-ΔN does not change the number of Zscan4⁺ cells. The results indicate that Zscan4c-ΔC functions in a manner similar to Zscan4-ERT2 (Tmx− condition).

Zscan4-ERT2 Enhanced and Prolonged Developmental Potency of ES Cells in the Absence of Tmx

To assess the effects of Zscan4-ERT2 on the developmental potency of ES cells, various ES cells were injected into mouse blastocysts, transferred to uteri, and their development was followed. The extent of ES cell potency was assessed by the percent chimerism in the pups based on coat colors: high (>70% chimerism), moderate (40%70%), low (<40%), and albino (0%) (FIG. 3A).

A V6.5 parental ES cell line at its early passage (p15) showed 18% high, 29% moderate, and 41% low chimerism, which are within the standard range for F1 hybrid ES cell lines. It is known that the developmental potency of ES cells generally becomes lower after multiple passages and/or plasmid transfection/drug selection. As expected, compared to a V6.5 parental ES cell line, a control V6.5 #2 ES cell line, which did not carry Zscan4-ERT2 but was generated after transfection and drug selection, showed a slightly lower overall potency, which was further reduced over multiple passages (p21, p23, and p30) (FIG. 3B). By contrast, V6.5 ZERT2 #18 ES cells showed much higher developmental potency than parental V6.5 and control V6.5 #2 ES cells: 73% high and 27% moderate chimerism at passage 19 (FIG. 3B). Even more surprising was that such a high level of potency was maintained for an extended period of time and passages: for example, even at passage 30, more than 40% of pups derived from V6.5 ZERT2 #18 ES cells showed “high” chimerism, whereas none of the pups derived from control V6.5 #2 ES cells showed “high” chimerism (FIG. 3B). Five other ES cell lines of different genetic backgrounds and transgenes were tested, including a very early passage line from freshly isolated ES cells (TA1). Potency-wise none of these ES cell lines could even come close to V6.5 ZERT2 #18 cell lines (FIG. 3B).

Interestingly, the exposure to Tmx for 2 to 3 days lowered the potency of both V6.5 #2 and V6.5 ZERT2 #18 ES cells relative to that in the Tmx⁻ condition, although the V6.5 ZERT2 #18 ES cells still showed higher potency than V6.5 #2 ES cells (FIG. 3B). These results seem to be consistent with the observation made by the global expression profiling (FIG. 1F): Tmx⁺ conditions increased the expression of genes unrelated to naturally occurring Zscan4⁺ (i.e., Em⁺) state in V6.5 ZERT2 #18 ES cells.

Testing Developmental Potency of ES Cells by the 4N Complementation Assay

It is widely recognized that the ultimate test for developmental potency is to see if ES cells alone injected into tetraploid (4N) blastocysts become an entire mouse (Nagy et al., Development 110:815-821, 1990). Compared to early passage V6.5 ES cells reported previously, which has achieved 15-25% pups alive at term (Eggan et al., Proc Natl Acad Sci USA 98:6209-6214, 2001), V6.5 ES cells at passage 18 only produced 2% live embryos (FIG. 4A). By contrast, V6.5 ZERT2 #18 ES cells even at passage 19 showed a much higher success rate −43% live embryos (FIGS. 4A and 4C). Similarly, two other independent clones (V6.5 ZERT2 #7; V6.5 ZERT2#10) also showed a high success rate of producing live embryos when 10-15 ES cells were injected into 4N blastocysts (FIG. 4A).

To compare the high success rate of V6.5 ZERT2 #18 cells with those of the best ES cells possible, freshly isolated ES cells were established from blastocysts with the same genetic background—F1 hybrid of C57BL/6J×12956/SvEvTac and were cultured in the best conditions currently available (Wong et al., Methods Enzymol 476:265-283, 2010) (FIG. 10 and FIG. 11). Of 20 blastocysts, 19 formed outgrowths in vitro, 13 of which continued to be cultured for an additional 7 days to form ES cell colonies, resulting in newly established ES cell lines (FIG. 10). Six clones out of 13 ES cell lines at the earliest passages (p3) were tested for their potency by injecting 10-15 ES cells into 4N blastocysts: one ES line, named “TA1”, showed the highest efficiency (60%) of producing live embryo at E13.5 (FIG. 4A and FIG. 11). Overall, these results obtained by the 4N complementation assays indicate that the developmental potency of V6.5 ZERT2 #18 ES cells even at the higher passage number is comparable to that of freshly isolated early passage ES cells.

To exclude the possibility that Zscan4-ERT2 affects only F1 hybrid ES cell lines, MC2 ZERT2 #6 ES cells were generated by transfecting a Zscan4-ERT2 plasmid to an MC2 ES cell line (C57BL/6J) (Olson et al., Cancer Res 63:6602-6606, 2003). Consistent with the reported low potency of C57BL/6J-derived ES cells (Brook et al., Proc Natl Acad Sci USA 94:5709-5712, 1997; Eggan et al., Proc Natl Acad Sci USA 98:6209-6214, 2001), both MC2 ES cells at passage 17 and genetically modified MC2 ES cells at passage 12-13 did not produce any live embryos (FIG. 4A). By contrast, MC2 ZERT2 #6 ES cells, which were cultured for more than 10 passages with the constitutive expression of Zscan4-ERT2, successfully achieved the production of 6% live embryos (FIG. 4A). The results thus suggest that the Zscan4-ERT2 construct can be used as a universal tool to enhance the developmental potency of pluripotent stem cells.

The unusually high developmental potency of V6.5 ZERT2 #18 cells prompted the further examination of the potency of single ES cells. It has been shown once that even a single ES cell can form a live pup, although the success rate is extremely low (1 mouse/192 injected blastocyst: 0.5%) (Wang and Jaenisch, Dev Biol 275:192-201, 2004). As expected from the fact that the same cell line was used as for the earlier study (Wang and Jaenisch, Dev Biol 275:192-201, 2004), the injection of a single parental V6.5 ES cell at passage 18 into 4N blastocysts produced one live embryo (1%) (FIG. 4B). Furthermore, single control V6.5 #2 ES cells did not produce any live embryos after injecting them into 77 tetraploid blastocysts (FIG. 4B). By contrast, of 44 tetraploid blastocysts that received a single V6.5 ZERT2 #18 cell, 3 (7%) became complete embryos, 2 (5%) of which were alive at the time of dissection (FIGS. 4B and 4D). This unusually high level of potency for V6.5 ZERT2 #18 ES cells was indeed comparable to that of early passage TA1 ES cells with 4% live embryos (FIG. 4B).

Discussion

It is disclosed herein that the constitutive presence of Zscan4-ERT2, without its usual activator Tmx, can increase the frequency of endogenous Zscan4 activation, resulting in the increase of developmental potency of ES cells. ES cells cultured in the accelerated Zscan4 activation cycle show improved chimerism and potency, which are demonstrated by high contribution to chimeric mice and efficient production of a whole mouse from a single ES cell.

How does the frequent activation of Zscan4 enhance and prolong the developmental potency of ES cells? Previously, it was demonstrated that the immortality of ES cells is maintained by an intermittent activation of Zscan4 (Zalzman et al., Nature 464:858-863, 2010). The shRNA-mediated continuous repression of Zscan4 makes ES cells undergo culture crisis after multiple cell passages (Zalzman et al., Nature 464:858-863, 2010). It is thus conceivable that even in their regular proliferating condition ES cells gradually lose their potency, which is rapidly restored by the transient activation of Zscan4 (Zalzman et al., Nature 464:858-863, 2010). Consistent with the notion that drastic changes, including rapid telomere extension by telomere sister chromatid exchange (Zalzman et al., Nature 464:858-863, 2010), are occurring in ES cells in Zscan4⁺ state, Zscan4⁺ cells (Em⁺ cells in the experiments described herein) did not produce chimeric animals when injected into blastocysts. In standard ES cells, the interval of transient Zscan4 activation may be longer than ideal; thus, ES cells steadily lose their average potency, irrespective of the occasional activation of Zscan4 (FIG. 4E, upper panel). More frequent activation of Zscan4 by the presence of Zscan4-ERT2 may maintain or even increase ES cell potency (FIG. 4E, lower panel).

Activation of endogenous Zscan4 by Zscan4-ERT2 without Tmx was unexpected, because ERT2-fusion proteins usually require Tmx for their activation. It is speculated that this may be related to a partial blocking of Zscan4 function, because the ERT2 domain is fused to the C-terminus of Zscan4, near four zinc-finger (C2H2) domains, whereas a SCAN domain is located at the N-terminus (Falco et al., Dev Biol 307:539-550, 2007). Considering the fact that Zscan4 should not be constitutively active in ES cells, the unexpected finding of Zscan4-ERT2 function provides an ideal means to increase the intermittent activation of endogenous Zscan4 expression. Irrespective of the mechanism, the presence of Zscan4-ERT2 in ES cells has beneficial effects on the potency of ES cells in long-term culture.

In view of the many possible embodiments to which the principles of the disclosure may be appliefd, it should be recognized that the illustrated embodiments are only examples of the disclosure and should not be taken as limiting the scope of the disclosure. Rather, the scope of the disclosure is defined by the following claims. We therefore claim all that comes within the scope and spirit of these claims.

Example 3 Overexpression of Zscan4c Alone can Rejuvenate ES Cells

To test whether transient overexpression of Zscan4 itself (i.e., unmodified Zscan4 protein) can increase the developmental potency of ES cells, we made a PB-tetZsan4c-IRES-beta-geo vector, in which the expression of the Zscan4c ORF is driven by the Dox-inducible tetO promoter (FIG. 12). The vector also contains beta-geo, G418-resistant gene, so that only the ES cells that contain Dox-inducible Zscan4c vector can be selected in the presence of G418. This piggyBAC vector was cotransfected with PB-CAG-rtTA vector (Dox-transactivator, which is necessary for the Dox-indelibility of teO promoter) and PcyL43 transposase vector (an enzyme that facilitates the integration of piggyBAC vectors into the genome). After the transfection, cells were cultured in the presence of G418 and Dox+ for 6 days, and then cultured in the absence of Dox subsequently. These cells were named V6.5 tetZscan4 ESC. As a control, parental V6.5 ES cells were used. The expression of Zscan4 can be transiently increased by adding Dox in the culture media (shown in blue box, FIG. 12).

These cells were cultured and passaged every 3 days. At certain passages, these cells were injected into tetraploid (4N) blastocysts to see whether these cells can form live mouse embryos at E13.5. The percent fraction of live embryos out of the number of injected blastocysts represents the developmental potency of ES cells (y-axis of FIG. 12).

As expected, control V6.5 ES cells showed the highest developmental potency (3%) at the early passage (passage 12), which declined gradually over multiple passages (FIG. 12). At the passage 24, control V6.5 ES cells completely lost their potency. By contrast, V6.5 tetZscan4c ES cells showed the increase of developmental potency after the transient Zscan4 overexpression from 3% (passage 12) to 9% (passage 18). When cells began to lose their developmental potency, we added Dox to the culture medium and transiently overexpressed Zscan4. As shown in FIG. 12, the transient overexpression of Zscan4 was able to increase the developmental potency of ES cells. Subsequently, we were able to show that by occasionally overexpressing Zscan4, ES cells can main their developmental potency even after long term cell culture (tested up to 37 passages).

These data clearly demonstrated that the transient overexpression of Zscan4 alone can increase the developmental potency (i.e., rejuvenate) ES cells. 

The invention claimed is:
 1. An expression vector comprising a nucleic acid molecule encoding a Zscan4-ERT2 fusion protein operably linked to a promoter, wherein the Zscan4-ERT2 fusion protein consists of the amino acid sequence of SEQ ID NO: 22 or SEQ ID NO:
 23. 2. An isolated cell comprising the expression vector of claim
 1. 3. The isolated cell of claim 2, which is a stem cell.
 4. The isolated cell of claim 3, wherein the stem cell is an embryonic stem (ES) cell or an induced pluripotent stem (iPS) cell.
 5. The isolated cell of claim 3, wherein the stem cell is a mouse, rat, human or non-human primate stem cell.
 6. A composition comprising the expression vector of claim 1 and a pharmaceutically acceptable carrier.
 7. The expression vector of claim 1, wherein the Zscan4-ERT2 fusion protein consists of the amino acid sequence of SEQ ID NO:
 23. 8. The expression vector of claim 1, wherein the Zscan4-ERT2 fusion protein consists of the amino acid sequence of SEQ ID NO:
 22. 9. An expression vector comprising a nucleic acid molecule encoding a mutant Zscan4-AC protein operably linked to a promoter, wherein the mutant Zscan4-AC protein consists of the amino acid residues (i) 1-311 of SEQ ID NO:2, (ii) 1-339 of SEQ ID NO: 2, (iii) 1-367 of SEQ ID NO: 2, (iv) 1-395 of SEQ ID NO: 2, (v) 1-394 of SEQ ID NO: 8, (vi) 1-423 of SEQ ID NO: 8, (vii) 1-451 of SEQ ID NO: 8, or (viii) 1-479 of SEQ ID NO:
 8. 10. An isolated cell comprising the expression vector of claim
 9. 11. The isolated cell of claim 10, which is a stem cell.
 12. A composition comprising the expression vector of claim 9 and a pharmaceutically acceptable carrier.
 13. The expression vector of claim 9, wherein the mutant Zscan-ΔC protein consists of the amino acid sequence of SEQ ID NO:
 25. 14. The expression vector of claim 9, wherein the mutant Zscan4-AC protein consists of the amino acid residues (i) 1-311 of SEQ ID NO:2, (ii) 1-339 of SEQ ID NO: 2, (iii) 1-367 of SEQ ID NO: 2, or (iv) 1-395 of SEQ ID NO:
 2. 15. The expression vector of claim 9, wherein the mutant Zscan4-AC protein consists of the amino acid residues (v) 1-394 of SEQ ID NO: 8, (vi) 1-423 of SEQ ID NO: 8, (vii) 1-451 of SEQ ID NO: 8, or (viii) 1-479 of SEQ ID NO:
 8. 